Computer networking
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Computer Networking. Bits and Bytes. Putting information into a form that a computer can deal with… “A” = 01000001 “B” = 01000010. Information “Encoding”. … 065 01000001 A 066 01000010 B 067 01000011 C 068 01000100 D 069 01000101 E 070 01000110 F 071 01000111 G ….

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Computer networking

Computer Networking


Bits and bytes

Bits and Bytes

  • Putting information into a form that a computer can deal with…

  • “A” = 01000001

  • “B” = 01000010


Information encoding

Information “Encoding”

  • 065 01000001 A

  • 066 01000010 B

  • 067 01000011 C

  • 068 01000100 D

  • 069 01000101 E

  • 070 01000110 F

  • 071 01000111 G


Review maybe

Review (maybe)

  • Have a bit…

    • 0 or 1

  • Take a whole byte…

    • Eight bits

    • R represents a letter or numeral or punctuation mark


Transmission of information

Transmission of Information

  • Bandwidth

    • Bits per second

    • Kilo

    • Mega

    • Giga


A computer network

A Computer Network

  • What is a computer network?

    A network is a collection of computers or computer-like devices that can communicate across a common transmission medium.


A computer network1

A Computer Network

  • In a network, requests and data from one computer pass across the transmission medium (which might be a network cable or a phone line) to another computer.

  • Example: four node network


A computer network2

A Computer Network

  • A computer interacts with the world through one or more applications (software) that perform specific tasks and manage input and output.

  • If that computer is part of a network, then some of those applications must be capable of communicating with applications on other network computers.


A computer network3

A Computer Network

  • A network protocol is a system of common rules that helps to define the complex process of transferring data. The data travels from an application on one computer, through the computer’s network hardware, across the transmission medium to the correct destination, and up through the destination computer’s network hardware to a receiving application.


Computer network

Application Software

Application Software

Network software within the operating system

Network software within the operating system

Network Interface Card (NIC)

Network Interface Card (NIC)

Computer Network

Transmission medium


A computer network4

A Computer Network

  • A network is usually described as being a local area network (LAN) or a wide area network (WAN)


Local area network lan

Local Area Network (LAN)

  • Many types of LAN technologies have existed over the years

  • One predominant LAN technology exists today - Ethernet


Ethernet

Ethernet

  • Contention media access method

  • Allows many computers on the same network to share the same bandwidth (basically share a common medium or connection)

  • Easily scalable – easy to improve and incorporate new technology as it becomes available


Ethernet1

Ethernet

  • Uses Carrier Sense Multiple Access with Collision Detect (CSMA/CD)

  • CSMA/CD is a protocol designed to allow multiple computers to share the network medium successfully

  • Designed to manage collisions


Ethernet2

Ethernet

  • What is a collision?

  • (example of four node 10Base-2 network)

  • All computers share the connection

  • Only one can transmit at a time

  • Suppose computer C is transmitting information to computer D

  • C “takes over” the wire – sends electrical signals onto the wire


Ethernet3

Ethernet

  • All computers on the network detect the transmission

  • Only D will process the transmitted data because C has addressed the information to D

  • A collision will occur if two computers attempt to transmit at the same time (like a group of people talking at a party)

B


Ethernet4

Ethernet

  • CSMA/CD – if a transmitting computer detects another computer attempting to transmit, it sends out a long “jam” signal that causes all computers on the network to be silent

  • A “back off” scheme is used to figure out who gets to transmit first


Ethernet5

Ethernet

  • On a busy Ethernet network collisions can be a big problem

  • SLOW!


Types of ethernet

Types of Ethernet

  • Ethernet was initially developed by Digital Equipment Corporation, Intel, and Xerox

  • The IEEE took their design and created the official network standard

  • The IEEE called this standard 802.3

  • 802.3 is the family name for all wired Ethernet types


Types of ethernet 10base2

Types of Ethernet – 10Base2

  • 10Mbps

  • Baseband technology

  • 185 meters (length) – almost 200 meters

  • 30 devices per segment

  • Uses coaxial cable (coax), BNC and T-connectors to connect to a network

  • Referred to as thinnet


Types of ethernet 10base5

Types of Ethernet – 10Base5

  • 10Mbps

  • 500 meters (length)

  • Up to 2500 meters with repeaters

  • Up to 1024 devices for all segments

  • Uses a large (thick) coaxial cable

  • Referred to as thicknet


Types of ethernet 10baset

Types of Ethernet – 10BaseT

  • 10Mbps

  • Uses Category 3 UTP wiring (phone wire)

  • Each device connects to a hub or switch

  • Only one device per segment (or wire)

  • Uses RJ-45 connectors

  • Supports a star topology


Types of ethernet 100baset x

Types of Ethernet – 100BaseT(X)

  • 100Mbps

  • Uses Category 5,6, or 7 UTP wiring

  • Up to 100 meters (length)

  • Only one device per segment (or wire)

  • Uses RJ-45 connectors

  • Supports a star topology


Types of ethernet 100basefx

Types of Ethernet – 100BaseFX

  • 100Mbps

  • Uses fiber optic cabling

  • Up to 412 meters (length)

  • Used for point-to-point connections

  • Uses ST or SC connectors


Types of ethernet 1000baset

Types of Ethernet – 1000BaseT

  • 1000Mbps

  • Up to 100 meters (length)

  • Category 5, 6, or 7 UTP wiring

  • Only one device per segment (or wire)

  • Uses RJ-45 connectors

  • Supports a star topology


Types of ethernet 1000basesx

Types of Ethernet – 1000BaseSX

  • 1000Mbps

  • Uses fiber optic cabling

  • Up to 550 meters (length) depending upon the size of the cable

  • Uses a 850 nanometer laser

  • Uses ST or SC connectors


Types of ethernet 1000baselx

Types of Ethernet – 1000BaseLX

  • 1000Mbps

  • Uses fiber optic cabling (multi-mode or single-mode)

  • Up to 10 kilometers depending on type of cable used

  • Uses a 1300 nanometer laser


Ethernet addressing

Ethernet Addressing

  • Media Access Control (MAC) address is stored on every Ethernet network interface card

  • 48 bits long (6 bytes)

  • Unique for each network interface card made (hopefully)


Ethernet addressing1

Ethernet Addressing

  • This computer: MAC = 00-02-2D-6D-CD-9B (base 16)

  • In binary: 00000000-00000010-00101101-01101101-11001101-10011011


Ethernet frames

Ethernet Frames

  • Ethernet divides data to be transmitted into frames

  • Ethernet frame has six parts:

    • Preamble (8 bytes)

    • Destination MAC address (6 bytes)

    • Source MAC address (6 bytes)

    • Type or length (2 bytes)

    • Data (64 – 1500 bytes) (usually)

    • FCS (4 bytes)


10base2 10base5

10Base2, 10Base5

  • Good news - no devices needed to control traffic on the network

  • Bad news – no devices available to control traffic on the network


Ethernet star topology

Ethernet (Star Topology)

  • 10BaseT, 100BaseT(X), 1000BaseT, 1000BaseSX, 1000BaseLX…

  • Require a device at center of star

  • Ethernet hub –or– Ethernet switch


Ethernet hubs and switches

Ethernet Hubs and Switches

  • Hub – any frames transmitted by a connected computer are sent out all ports (to all connected computers)

  • Switch – “learns” which computers are connected, what port they are connected to, and only transmits frames out the port that the specific receiving computer is connected to


Hubs switches collisions

Hubs, Switches, Collisions

  • Consider a 4-node 10Base2 network, a 4-node 10BaseT network with a hub, and a 4-node 10BaseT network with a switch:

  • Which network will have the most collisions? the least?


Hubs switches collisions1

Hubs, Switches, Collisions

  • A network with a hub is a single collision domain (bad!)

  • A network with a switch has a separate collision domain for each port (good!)


Ethernet hubs and switches1

Ethernet Hubs and Switches

  • Hubs – single collision domain, single broadcast domain

  • Switches – multiple collision domains, single broadcast domain

  • Hubs and switches can be used together in a network


Ethernet broadcasts

Ethernet Broadcasts

  • A broadcast frame has destination address of FF-FF-FF-FF-FF-FF (binary all 1’s)

  • A switch will send broadcast frames out every port (except the one on which the frame was received)


Ethernet broadcasts1

Ethernet Broadcasts

  • Broadcasts are sometimes necessary

  • Broadcasts are sometimes evil


Broadcast domains

Broadcast Domains

  • Example: Consider an Ethernet network with an 8-port switch fully connected: How many broadcasts domains are in this network? How many collision domains are in this network?


How does a switch work

How Does A Switch Work?

  • It records the source MAC address in every frame it receives and stores it in the filter table with the associated port from which it came

  • If a switch receives a frame destined for a MAC address that is not in the filter table, the switch will send it out every port


Real world show and tell

Real World Show and Tell

  • HP Procurve 2848 switch

    • Can mix Ethernet standards on one device

      • 1000Base-LX or 1000Base-SX

      • 1000Base-T/100Base-T/10Base-T autosensing


That s a wrap on ethernet for now

That’s a Wrap on Ethernet(for now)

  • Other LAN technologies:

    • FDDI (Fiber Distributed Data Interface)

    • Token Ring

    • LocalTalk (Apple)


Remember this let s refine it

Application Software

Application Software

Network software within the operating system

Network software within the operating system

Network Interface Card (NIC)

Network Interface Card (NIC)

Remember This?(Let’s refine it…)

Transmission medium


Network layers osi model open systems interconnection

Application Software

Network software within the operating system

Network Interface Card (NIC)

Network Layers – OSI Model(Open Systems Interconnection)

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer


Why all the layers

Why All the Layers?

  • Provides a model for how communication should take place

  • Real world example – organization chart in a business (president, VP, mid-managers, low-managers, entry-level staff…)


Why all the layers1

Why All the Layers?

  • Software developers only have to be concerned with a particular layer’s functions

  • Allows many companies (vendors) to develop software that will work together

  • Allows various types of network hardware and software to communicate

  • Changes in one layer don’t cause problems in other layers


Role of each layer in osi model

Role of Each Layer in OSI Model

  • Application Layer – provides an interface between the application software (e.g. Internet Explorer, AIM…) and the lower network layers

  • Presentation Layer – translates data to standard format; provides encryption and data compression


Role of each layer in osi model1

Role of Each Layer in OSI Model

  • Session Layer – “directs traffic”

    (will not be emphasized – just know that it exists and where)


Role of each layer in osi model2

Role of Each Layer in OSI Model

  • Transport Layer

    • takes streams of data from application software and upper layers

    • converts data stream into segments

    • opens communication with receiving computer

    • Provides either “reliable” or “unreliable” communication to receiving computer


Role of each layer in osi model3

Role of Each Layer in OSI Model

  • Network Layer

    • Manages network addresses

    • Responsible for transporting data to other computers which may not be attached to the local area network

    • Takes segments from transport layer

    • Sends datagrams (or packets) to data link layer


Role of each layer in osi model4

Role of Each Layer in OSI Model

  • Data Link and Physical Layers

    • This is where Ethernet exists

    • Data link layer takes datagrams from network layer and builds frames

Preamble

(8 bytes)

Data

(64 up to

1500 bytes)

Source MAC

Address

(6 bytes)

Length

(2 bytes)

FCS

(4 bytes)

Destination MAC

Address

(6 bytes)


Data encapsulation through layers

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Data Encapsulation Through Layers

  • Information from layer above is encapsulated (has a header and error detection information added)

  • Corresponding layer on receiving computer uses and then removes the header and error detection data (if any)

  • More on this later…


Tcp ip

TCP/IP

  • Transport and Network Layers Protocols

  • TCP – Transmission Control Protocol

    • Operates at the transport layer (layer 4)

  • IP – Internet Protocol

    • Operates at the network layer (layer 3)


Tcp ip1

TCP/IP

  • Developed by Department of Defense in 1960s

  • Wanted to connect mainframe and supercomputers in different parts of the country


Tcp ip2

TCP/IP

  • Wanted the network to not have a single point of failure

  • End node verification

  • Dynamic routing


Tcp ip3

TCP/IP

  • This network was called ARPAnet (Advanced Research Projects Agency)

  • NSF took the design and used it to connect research centers and universities

  • NSF’s network became known as the Internet (Al Gore??)


Features of tcp ip

Features of TCP/IP

  • Logical addressing

    • Ethernet can’t get us very far!

  • Routing (new network device for us!)

    • Routers connect networks together

    • Data addressed to the local network doesn’t go through the router


Ip addresses

IP Addresses

  • 32 bit (4 bytes)

  • Usually displayed in base 10 notation

  • Example: 12.146.244.182

  • Unique to each computer (but user controllable)


Ip addresses1

IP Addresses

  • Network portion

  • Host (or computer portion)

  • Telephone number analogy

  • Subnet mask (netmask) determines boundary


Example ip address

Example – IP Address

  • Example IP Address = 206.74.226.4

    binary (base 2) equivalent = 11001110.01001010.11100010.00000100

  • Netmask = 255.255.255.0

    binary equivalent = 11111111.11111111.11111111.00000000

    - The “1”s (the “on” bits) indicate the network portion, the “0”s represent the host (or computer) portion


Ip address network

IP Address - Network

  • An address with all zero’s in host portion is generally referred to as the network address.

  • Example:

    206.74.226.0

    11001110.01001010.11100010.00000000


Ip address broadcast

IP Address - Broadcast

  • An address with all ones in host portion is the broadcast address.

  • Example:

    206.74.226.255

    11001110.01001010.11100010.11111111


A rule or two

A Rule or Two

  • A host cannot have the network address.

  • A host cannot have the broadcast address.

    (Basically, an IP address assigned to a host can’t have all ones or all zeros in the host portion of the address.)

  • 127.0.0.1 is reserved.


Ip addresses2

IP Addresses

  • Three main classes of IP addresses:

    • Class A

    • Class B

    • Class C


Ip addresses class a

IP Addresses – Class A

  • Class A

    • Intended for the networks with very large number of nodes

    • First byte of address (first octet) is network portion (i.e. netmask = 255.0.0.0)

    • First bit of first byte of address must be 0 (binary)

    • What is the range of network addresses?

    • How many networks?

    • How many hosts?


Ip addresses class b

IP Addresses - Class B

  • Class B

    • Intended medium-sized networks

    • First two bits of first byte of address must be 10 (binary)

    • First two bytes of address (first two octets) are network portion (i.e. netmask = 255.255.0.0)

    • What is the range of network addresses?

    • How many networks?

    • How many hosts?


Ip addresses class c

IP Addresses - Class C

  • Class C

    • Intended for smaller networks

    • First three bits of first byte of address must be 110 (binary)

    • First three bytes of address (first three octets) are network portion (i.e. netmask = 255.255.255.0)

    • What is the range of network addresses?

    • How many networks?

    • How many hosts?


Ip address classes summary

IP Address Classes - Summary

  • Class A

    • Network address range:

      • 0.x.x.x – 126.x.x.x (127 class A addresses)

    • Netmask = 255.0.0.0

  • Class B

    • Network address range:

      • 128.0.x.x – 191.255.x.x (16384 class B addresses)

    • Netmask = 255.255.0.0

  • Class C

    • Network address range:

      • 192.0.0.x – 223.255.255.x (2,097,152 class C addresses)

    • Netmask = 255.255.255.0


Ip addresses class d and e

IP Addresses (Class D and E)

  • They exist

  • Not commonly used

  • We will not study them


Why is the netmask needed

Why Is the Netmask Needed?

  • If we can look at the first octet in the address and tell which class the address is in, why do we need to specify the netmask?

  • Answer: The netmask can be varied to allow “subnetting”, more later…


Review the big picture

Review the Big Picture

  • Application software

  • Network layers (OSI model)

    • Application, Presentation, Session -> upper layers

    • Transport layer (TCP is the transport layer protocol we are studying)

    • Network layer protocol (IP)

    • Data Link and Physical layers (Ethernet)


Review the big picture1

Review the Big Picture

  • Upper layers produce data stream

  • TCP (transport layer protocol)

    • takes data

    • produces segments

    • sends segments to network layer protocol

  • IP (network layer protocol takes segments)

    • Constructs a packet

    • puts segment into data field in packet

    • adds IP header (with source and destination IP addresses and other info)

    • sends packet down to data link layer

  • Ethernet (data link layer)

    • Constructs a frame

    • puts IP Packet into data field in frame

    • adds header and FCS fields to frame

    • sends frame to physical layer (network interface card)

  • Physical layer sends the frame onto the medium (the wire) as series of bits in the form of electrical signals


Ip packet a k a ip datagram

IP Packet (a.k.a. IP datagram)

  • Version

    • IP version number

    • 4 bits

  • Header length

    • 4 bits

  • Priority and type of service

    • 8 bits

  • Total length

    • Length of header and data combined (entire packet)

    • 16 bits

  • Indentifier

    • Like a serial number for the packet

    • 16 bits


Ip packet

IP Packet

  • Flags

    • Indicates fragmentation

    • 3 bits

  • Fragmentation

    • If packet is too large for frame, provides info to help reassemble packet on other end

    • 13 bits

  • Time To Live

    • Expiration time

    • 8 bits

  • Protocol

    • Transport layer info (port number and protocol)

    • 8 bits

  • Header checksum

    • For error detection within IP packet

    • 16 bits


Ip packet1

IP Packet

  • Source IP address

    • 32 bits (of course!)

  • Destination IP address

    • 32 bits

  • Options

    • Used for testing, debugging, etc.

    • 0 bits or 32 bits

  • Data

    • The “payload” - contains the data from/to the transport layer (usually the TCP segment)

    • Varies in length


Ip packet2

IP Packet

  • Most important things to remember:

    • Contains source and destination IP addresses

    • Contains TCP port info

    • Contains data


Examining incoming data

Examining Incoming Data

  • Examine FCS field in frame

  • Examine destination MAC address in frame

  • Examine header checksum in IP packet

  • Examine destination IP address in packet

  • If all these pass…

  • Send data (TCP segment) to TCP for further processing


Subnetting example

Subnetting Example

  • Suppose you have a small office network with only 5 computers/network devices (5 hosts). Assigning a class C license to you organization would be wasteful of the precious IP addresses.


Subnet example

Subnet Example

  • Your ISP could assign you an network IP address like this:

    • Network address = 220.178.12.144

      • Binary = 11011100.10110010.00001100.10010000

    • Netmask = 255.255.255.240

      • Binary = 11111111.11111111.11111111.11110000

    • Broadcast = 220.178.12.?

      • Binary = ?

  • How many hosts can be on this IP subnet?


What good is ip subnetting

What good is IP subnetting?

  • Conserves addresses

  • Allows a large network to be broken up into smaller networks to increase efficiency:

    • Reduce the broadcasts that hosts receive

    • Problems can be contained (broadcast storms)

    • Allow network bandwidth to be controlled


How do we subnet

How do we subnet?

  • Router

    • Connected to two or more subnetworks

    • Forwards packets based on destination IP address

  • Each network interface on a router will have an IP address assigned to it that is part of the IP subnet


Review switching broadcasts collisions

Review Switching, Broadcasts, Collisions

  • Hubs repeat everything

  • Switches forward frames based on destination MAC (Ethernet) address

  • Switches always forward broadcasts

  • Every switch port is a collision domain


Back to routing

Back to Routing…

  • Routers

    • Do NOT forward Ethernet broadcasts

    • Do forward IP packets based on destination IP address

    • Forward a packet to the network in which the destination IP address resides


Routing example 1

Routing Example #1

  • Consider Computer A and Computer B directly connected via Ethernet cable (Wow, you can do that?)

  • Computer A sends data to Computer B

  • What happens?


Back to routing example 1 cont d

Back to Routing Example #1 (cont’d)

  • Computer A: 220.178.12.42

  • Computer B: 220.178.12.43

  • The netmask is 255.255.255.0

    (Remember: all hosts on a common subnet must have a common netmask and network address!)

  • What is the network address for this small network?


Back to routing example 1 cont d1

Back to Routing Example #1 (cont’d)

  • In Computer A…

    • Data comes from app s/w and upper layers

    • TCP creates a segment, passes down to IP

    • IP builds packet with destination IP address, source IP address, data (TCP segment), and other header fields

    • IP determines if the destination IP address is on the same subnet as the source IP address (why? more on this in routing example #2)

    • IP passes packet down to data link layer for frame creation… but wait!


Back to routing example 1 cont d2

Back to Routing Example #1 (cont’d)

  • The Ethernet frame must have a destination MAC address, right?

  • No data can be passed from A to B on an Ethernet network without a destination MAC address

  • What gives??


Address resolution protocol arp to the rescue

Address Resolution Protocol (ARP) to the rescue…

  • If IP has a packet to send, it must inform the data link layer (Ethernet) of the destination MAC address

  • ARP serves as IP’s detective

  • IP uses ARP to find the MAC address that corresponds to a particular IP address


Address resolution protocol arp

Address Resolution Protocol (ARP)

  • ARP sends out an Ethernet broadcast frame (destination address is all “1” in binary or all “FF” in hexadecimal)

  • The broadcast frame basically asks, “Would the host with this IP address please respond to me with your MAC address?”

  • All hosts on the subnet will process the frame – only the particular host with the destination IP address will respond


Back to routing example 1 cont d3

Back to Routing Example #1 (cont’d)

  • After “ARPing” for the MAC address, IP sends the packet down to the data link layer along with the destination MAC address

  • Data link layer builds the frame

  • Passes to physical layer for transmission as series of bits… yada yada yada


Computer networking

Let’s do it again…


Routing example 2

Routing Example #2

  • Router in between Computer A and Computer B:

    • 220.178.12.0, netmask 255.255.255.0

    • 220.178.13.0, netmask 255.255.255.0

  • Assume router interfaces have following IP addresses/netmasks:

    • E0: 220.178.12.1 / 255.255.255.0

    • E1: 220.178.13.1 / 255.255.255.0


Routing example 2 cont d

Routing Example #2 (cont’d)

  • Computer A sending data to Computer B

  • Computer A

    • IP address = 220.178.12.34

    • Netmask = 255.255.255.0

  • Computer B

    • IP address = 220.178.13.147

    • Netmask = 255.255.255.0

  • Question: What are the network and broadcast addresses for the two subnets in this example?


Routing example 2 cont d1

Routing Example #2 (cont’d)

  • In Computer A…

    • Data comes from app s/w and upper layers

    • TCP creates a segment, passes it down to IP

    • IP builds packet with destination IP address, source IP address, data (TCP segment), and other header fields

    • IP determines if the destination IP address is on the same subnet as the source IP address

    • If destination is on the same subnet, then ARP for the MAC address of computer with destination IP address


Routing example 2 cont d2

Routing Example #2 (cont’d)

  • But wait! In this example, Computer B is NOT on the same subnet with Computer A

  • Will ARP work? – Remember that the router does not forward Ethernet broadcasts and ARP uses an Ethernet broadcast…


Routing example 2 cont d3

Routing Example #2 (cont’d)

  • Computer A must know IP address of default gateway for its subnet

  • The default gateway is the IP address of the router interface on that subnet


Routing example 2 cont d4

Routing Example #2 (cont’d)

  • Computer A…

    • “ARPs” for the MAC address of the default gateway (the router)

    • Router responds with MAC address for its Ethernet interface on that subnet (E0)

    • Computer A sends Ethernet frame to router (containing the IP packet with the original source and destination address)


Routing example 2 cont d5

Routing Example #2 (cont’d)

  • Router…

    • Sees the frame is for him

    • The router’s data link layer passes the IP packet up

    • The IP layer on the router examines the IP destination address

    • The router sees that the destination is on the same subnet with interface E1

    • “ARPs” for MAC address of destination computer (Computer B) – Computer B responds

    • Router builds a frame with recipients real MAC address as destination and original IP packet payload

    • Sends the frame down to physical layer for transmission


Ip addressing subnetting review

IP Addressing/Subnetting Review

  • Example:

    • IP Network Address: 196.24.44.80

    • Subnet Mask (netmask): 255.255.255.248

  • What is the range of host addresses?

  • What is the broadcast address?


Ip addressing subnetting review1

IP Addressing/Subnetting Review

  • Network Address:

    • 11000100.00011000.00101100.01010000

  • Netmask:

    • 11111111.11111111.11111111.11111000

  • Broadcast:

    • 11000100.00011000.00101100.01010111

  • First host is network address + 1

    • 11000100.00011000.00101100.01010001

  • Last host is broadcast – 1

    • 11000100.00011000.00101100.01010110


  • Ip addressing subnetting review2

    IP Addressing/Subnetting Review

    • First host is network address + 1

      • 11000100.00011000.00101100.01010001

      • 196.24.44.81

  • Last host is broadcast – 1

    • 11000100.00011000.00101100.01010110

    • 196.24.44.86

  • Range of host addresses on this subnet:

    • 196.24.44.81 -> 196.24.44.86


  • Routing example 3

    Internet

    Routing Example #3

    E0: 206.113.116.169

    C

    Router A

    Switch

    E2:

    221.19.10.1

    E1: 220.178.13.2

    E0: 220:178.13.1

    B

    Router B

    A

    Switch

    Switch

    E2:

    220.178.17.161

    E1:

    220.178.12.145


    Routing example 3 cont d

    Routing Example #3 (cont’d)

    • Computer A to send IP packet to Computer C


    Routing example 3 cont d1

    Routing Example #3 (cont’d)

    • Computer A…

      • “ARPs” for the MAC address of the default gateway (router A)

      • Router A responds with MAC address for its Ethernet interface on that subnet (E1)

      • Computer A sends Ethernet frame to router A (containing the IP packet with the original source and destination address)


    Routing example 3 cont d2

    Routing Example #3 (cont’d)

    • Router A…

      • Sees that the frame is for him (destination MAC address)

      • The router’s data link layer passes the IP packet up

      • The IP layer on the router examines the IP destination address

      • The router sees that the destination is NOT on any subnet to which he is connected

      • Router A discards (“drops”) the packet

      • The End


    Routing example 3 cont d3

    Routing Example #3 (cont’d)

    • How can this be made to work?

      • Solution #1: Configure a default route on router A

        • Similar to default gateway on computers

        • Default route is the IP address on a local subnet to which all packets destined for foreign IP addresses are forwarded


    Routing example 3 cont d4

    Routing Example #3 (cont’d)

    • Router A would have in its configuration:

      > Default route = 220.178.13.2

      (The IP address for E1 on router B)


    Routing example 3 cont d5

    Routing Example #3 (cont’d)

    • Now, what will router A do?

      • Sees that the frame is for him (destination MAC address)

      • The router’s data link layer passes the IP packet up

      • The IP layer on the router examines the IP destination address

      • The router sees that the destination is NOT on any subnet to which he is connected

      • Router A “ARPs” for MAC address corresponding to default route (gateway) address

      • Gets a reply, sends frame to E1 on router B


    Routing example 3 cont d6

    Routing Example #3 (cont’d)

    • Router B

      • Sees that the frame is for him

      • Unpacks the frame and sends data up to IP

      • IP sees that the destination IP address is on the same subnet with interface E2

      • “ARPs” for MAC address of destination computer (Computer B) – Computer B responds

      • Router builds a frame with recipients real MAC address as destination and original IP packet payload

      • Sends the frame down to physical layer for transmission


    Routing example 3 cont d7

    Routing Example #3 (cont’d)

    • Solution #2: Configure a static route on router A

      • Simply tells router A to send all packets destined for a particular foreign network to a specific local IP address

      • In this example, configure router A with following command:

        > 221.19.10.0 via 220.178.13.2


    Routing example 3 cont d8

    Routing Example #3 (cont’d)

    • What routing configuration does router A need to allow hosts full access to LAN hosts and the Internet?

    • What about router B?


    Ip routing summary

    IP Routing Summary

    • Default routing – the IP address on a local subnet to which all packets destined for foreign IP addresses are forwarded

    • Static routing – IP addresses on a local subnet to which all packets destined for particular foreign IP addresses are forwarded


    Ip routing summary1

    IP Routing Summary

    • Routing information in a router is contained in the routing table

    • Example of a routing table:

      192.168.50.0 255.255.255.0 connected to E0

      192.168.51.0 255.255.255.0 connected to E1

      192.168.40.0 255.255.255.0 via 192.168.50.1

      192.168.30.0 255.255.255.0 via 192.168.51.1

      0.0.0.0 via 192.168.51.1


    Dynamic routing

    Dynamic Routing

    • Routers “educate” each other about networks to which they are connected

    • A protocol for exchanging route information among routers is called a routing protocol


    Dynamic routing1

    Dynamic Routing

    • The most famous routing protocol in use is called Routing Information Protocol (RIP, very creative, huh?)

    • In RIP, a router will report all of the networks to which it is connected and also the number of “hops” (or routers) between it and the particular networks

    • Also propagates RIP info it has received


    Dynamic routing2

    Dynamic Routing

    • Upon receipt of RIP info from a neighboring router, all hop counts are incremented by 1 and the info is placed into the routing table


    Dynamic routing3

    Dynamic Routing

    • Example: 3 routers (next slide)


    Dynamic routing example rip

    Dynamic Routing Example (RIP)

    E0: 17.14.210.32

    E0: 206.79.211.44

    E1: 177.100.48.2

    E1: 192.168.21.32

    A

    E3: 192.168.21.33

    E1: 177.100.48.3

    B

    C

    E2: 12.34.25.147

    E2: 186.18.90.97

    (assume all netmasks are 255.255.255.0)


    Dynamic routing example rip1

    Dynamic Routing Example (RIP)

    • Router A reports to router B:


    Dynamic routing example rip2

    Dynamic Routing Example (RIP)

    • Why is the hop count important?

      • A router might receive route information for a particular network from two directions

      • When this happens, the router will only keep the route with the smallest hop count (closest path to the network)


    Dynamic routing example rip3

    Dynamic Routing Example (RIP)

    • Router B will add these entries in routing table:

      17.12.210.0 via 192.168.21.32

      12.34.25.0 via 192.168.21.32


    Dynamic routing example rip4

    Dynamic Routing Example (RIP)

    • Router B reports to router C:

    • What will router C add to its routing table?


    Dynamic routing4

    Dynamic Routing

    • RIP is being gradually being replaced by newer more efficient routing protocols

    • Open Shortest Path First (OSPF) is becoming prevalent


    Layers again

    Layers Again…

    • Upper layers (s/w application) -> Transport Layer

    • Transport -> Network -> Data Link -> Physical

    • Layers talk to their counterparts…

    • At what layers do routers operate?

    • How does the requirement for end node verification fit in?


    Layers again1

    Layers Again…

    • Transport layer is the first layer in which the end nodes really talk to each other

    • Transport layer is where “end node verification” takes place


    Transport layer layer 4

    Transport Layer (Layer 4)

    • An interface for network applications – provides a way for application software to access the network. The designers wanted a way to send data not just to a particular computer, but to a particular network application running on the destination computer


    Transport layer layer 41

    Transport Layer (Layer 4)

    • Provide multiplexing/demultiplexing – the transport layer must be capable of simultaneously supporting several network applications and directing data to the network layer

    • Provide mechanism for one network application to maintain connections with more than one computer


    Transport layer layer 42

    Transport Layer (Layer 4)

    • Error checking

      • Similar to network and data link layer error checking (nobody don’t trust nobody)

    • Flow control

      • One computer doesn’t allow the other computer to overwhelm it with data

    • Verification

      • Making sure all the data got delivered


    Transport layer layer 43

    Transport Layer (Layer 4)

    • Two transport layer protocols

      • Transport Control Protocol (TCP)

        • provides extensive error checking and flow control to ensure successful delivery of data

        • It is “connection-oriented”

      • User Datagram Protocol (UDP)

        • Provides very basic error checking

        • Reliability sacrificed for speed and efficiency

        • It is “connectionless”


    Transport layer layer 44

    Transport Layer (Layer 4)

    • Oversimplified example – two humans in connection-oriented conversation:

      Bill: Hello Larry. Are you listening? I have something to say.

      Larry: Yes, I’m listening Bill.

      Bill: There is…

      Larry: Yes, I understand.

      Bill: …a baseball game…

      Larry: Yes, I understand.

      Bill: … on Saturday.

      Larry: Yes, I understand.

      Bill: That’s all I have to say.

      Larry: Ok, I’ll stop listening to you.

      Bill: Ok, I’ll stop talking to you.


    Transport layer layer 45

    Transport Layer (Layer 4)

    • Oversimplified example – two humans in connectionless conversation:

      Bill: Larry, there is a baseball game on Saturday.


    Tcp and udp ports

    TCP and UDP Ports

    • Network software applications access the transport layer protocols through a port

    • Ports are numbered – only one software application can use one port number at a time

    • The ports are not real, hardware ports – they are software ports


    Tcp port example

    TCP Port Example

    • Example: Computer A wants to download a web page from computer B

      • Computer B’s web server software is accepting connections on TCP port 80

      • Computer A will pick an unused port number at random and open a connection to computer B on its port 80


    Tcp port example1

    1

    2

    3

    80

    65534

    1

    3

    65534

    2

    80

    UDP

    TCP

    TCP Port Example

    Computer B (web server)

    Web server software

    Network Layer (IP)

    Data Link Layer (Ethernet)

    Computer A


    Tcp port example2

    TCP Port Example

    • The web server software on B has notified TCP that it wishes to accept connections on port 80 (passive mode)

    • The browser software on computer A then asks TCP (on computer A) to open a connection to port 80 on computer B

    • Computer A will use a random port number not in use already


    Tcp port example3

    Computer A

    Browser software (Internet Explorer)

    1

    2

    3

    80

    65534

    1

    65534

    2

    3571

    UDP

    TCP

    Network Layer (IP)

    Data Link Layer (Ethernet)

    TCP Port Example

    Computer B

    (web server)


    Well known tcp ports

    Well Known TCP Ports

    • 20, 21 – FTP

      • File Transfer Protocol

    • 23 – Telnet

      • Terminal emulation interface

    • 25 – SMTP

      • Simple Mail Transfer Protocol

    • 53 – DNS

      • Domain Name Service

    • 80 – HTTP

      • Hypertext Transfer Protocol (the web)

    • 110 – POP3

      • Post Office Protocol (checking email)


    Tcp segment

    TCP Segment

    • Source port (16 bits)

      • Port number used by transmitting host (max 65534)

    • Destination port (16 bits)

      • Port number used by receiving host (max 65534)

    • Sequence number (32 bits)

      • Number corresponding to first byte of data it will send

    • Acknowledgement number (32 bits)

      • The next sequence number that the receiver is expecting

    • Data offset (4 bits)

      • Length of the header (integer multiple of 32 bits)


    Tcp segment1

    TCP Segment

    • Reserved (6 bits)

      • All zeroes, all the time

    • Control flags (1 bit each)

      • URG

      • ACK

      • PSH

      • RST

      • SYN

      • FIN


    Tcp segment2

    TCP Segment

    • Window (16 bits)

      • The next sequence number that the transmitting computer is free to send without further acknowledgement

    • Checksum (16 bits)

      • Error correction (similar to lower layers)

    • Urgent pointer (16 bits)

      • Basically, a sequence number at which some urgent data will begin

    • Options (variable length)

      • Usually either 0 bits or 32 bits

    • Padding (variable)

      • Extra zero bits to make sure the header is integer multiple of 32 bits

    • Data (variable length)


    Tcp segment most important fields

    TCP Segment – Most Important Fields

    • Source port

    • Destination port

    • Sequence number

    • Acknowledgement number

    • Window

    • Data


    Establishing a tcp connection three way handshake

    Establishing a TCP Connection(Three-Way Handshake)

    • From previous example

      • 1) Computer A sends a segment to computer B requesting “synchronization” – basically a request to open a connection (session)

        • This segment also contains A’s initial sequence number

      • 2) Computer B sends a segment back that acknowledges the synchronization and contains it’s initial sequence number


    Establishing a tcp connection three way handshake cont d

    Establishing a TCP Connection(Three-Way Handshake)(cont’d)

    3) Computer A acknowledges receipt of computer B’s initial sequence number


    Tcp flow control

    TCP Flow Control

    • The receiving computer, in order to prevent the transmitting computer from overwhelming it with data, uses the Window field is used to define how many bytes of data the transmitting computer can send before an acknowledgement


    Tcp flow control illustration a sending data to b

    TCP Flow Control (illustration)(A sending data to B)

    B

    A

    TCP Segments

    3 bytes of data (1,2,3)

    Acknowledge 4, window 5

    5 bytes of data (4,5,6,7,8)

    Acknowledge 6, window 2

    2 bytes of data (6,7)


    Tcp flow control1

    TCP Flow Control

    • It’s possible that segments will arrive at the receiving computer in the wrong order (order different than transmitted)

    • This may be due to a router going down and the route between the two computers being changed (dynamic routing)

    • TCP can put segments back in the correct order before giving data to application software


    Udp flow control

    UDP Flow Control (?)

    • None!

    A

    B

    8 bytes of data (1-8)


    Udp datagram

    UDP Datagram

    • Source port (16 bits)

      • Port number used by transmitting host (max 65534)

    • Destination port (16 bits)

      • Port number used by receiving host (max 65534)

    • Length (16 bits)

      • Length of the entire datagram

    • Checksum (16 bits)

      • Error detection

    • Data (varies)


    Firewalls

    Firewalls

    • Definition: Hardware and/or software designed to a provide security for a network or a particular computer

    • Can control access based on:

      • network layer (layer 3)

      • transport layer (layer 4)

      • Application s/w


    Typical firewall configuration as a standalone network device

    Typical Firewall Configuration(as a standalone network device)

    • Two network interfaces

      • Inside interface (trusted)

        • Usually connected to internal corporate/office/campus network

      • Outside interface (not trusted)

        • Usually connected to Internet (via Internet service provider


    Typical firewall configuration

    Typical Firewall Configuration

    • Hosts on the network on the inside interface usually have unrestricted ability to open TCP connections (and send UDP datagrams) to hosts on outside

    • Exceptions can occur:

      • Disallow access to certain web sites

      • Disallow email to be sent through external mail servers (virus/worm control)


    Typical firewall configuration1

    Typical Firewall Configuration

    • Hosts on the outside interface (the rest of the Internet) usually have no ability to open TCP connections or send un-requested UDP datagrams to hosts on inside network

    • Exceptions:

      • Allow external hosts to access a web server on port 80 (HTTP port)

      • Allow external hosts access to a mail server on port 25 (SMTP) for delivering email


    Typical firewall configuration nat

    Typical Firewall Configuration (NAT)

    • Most firewalls capable of Network Address Translation (NAT)

    • Allows for a private IP addressing scheme on the inside network

    • When inside hosts need to communicate with hosts outside, the firewall translates the inside (private) IP address to a real IP in outgoing IP packets


    Typical firewall configuration nat1

    Typical Firewall Configuration (NAT)

    • For an IP packet coming back from an outside host, the firewall will translate the destination IP address back to the particular host’s private (inside) address

    • When the session is over, the outside IP address can be “recycled” to be used for another inside host


    Typical firewall configuration nat2

    Typical Firewall Configuration (NAT)

    • Advantage: can allow a large number of hosts on inside network to share a relatively small number of “real” IP addresses for Internet use

    • Very important for home networks with more than one computer (together with PAT…more later)


    Typical firewall configuration nat3

    Typical Firewall Configuration (NAT)

    • Network address translations can be static so that an inside host will always have a particular outside (real) IP address

    • This is necessary for web servers, email servers, DNS servers, or any computer that may need to allow incoming connection requests


    Typical firewall configuration pat

    Typical Firewall Configuration (PAT)

    • Most firewalls capable of Port Address Translation (PAT)

    • Example: Inside host opens TCP connection with source TCP port 5612

      • Firewall will translate the TCP port number to something else in the outgoing TCP segment

      • Why?


    Typical firewall configuration pat1

    Typical Firewall Configuration (PAT)

    • Suppose you have a home network with four computers

    • All of you are browsing the web at the same time

    • Your firewall may be supporting a private address scheme on your home network

    • All of you may be sharing a single real IP address on the outside


    Typical firewall configuration pat2

    Internet

    Typical Firewall Configuration (PAT)

    PAT Global

    192.168.0.15

    Source addr

    Source addr

    10.0.0.2

    192.168.0.15

    Destination

    addr

    10.0.0.2

    Destination addr

    172.30.0.50

    172.30.0.50

    Source port

    49090

    Source port

    2000

    Destination

    port

    Destination port

    80

    80

    10.0.0.11

    Source addr

    Source addr

    192.168.0.15

    Destination

    addr

    Destination

    addr

    172.30.0.50

    172.30.0.50

    Source port

    49090

    Source port

    2001

    10.0.0.11

    Destination

    port

    80

    Destination

    port

    80


    Typical firewall configuration pat3

    Typical Firewall Configuration (PAT)

    • PAT allows multiple inside hosts to share a single outside IP address

    • NAT and PAT used together to conserve our precious IP addresses


    Computer networking

    DHCP

    • Dynamic Host Control Protocols

    • Used to automatically give networked computers IP configuration information

    • One or more computers on a network are usually dedicated to being DHCP servers

    • (see ipconfig and TCP/IP properties on a Windows computer)


    Dhcp general idea

    DHCP – General Idea

    • A computer connected to the network is turned on (and set to “Obtain IP address automatically”)

    • During the process of starting up, the computer sends out a broadcast asking for a DHCP server to send it IP configuration information (DHCP discover)

    • A DHCP server will respond with an IP address, subnet mask, default gateway info, and possibly much more (DHCP offer)


    Dhcp general idea1

    DHCP – General Idea

    • The computer will respond to the DHCP server with an acknowledgement that it has accepted the IP address and other info (DHCP ack)

    • The DHCP server keeps a database of the IP addresses it has handed out (leased) along with the computer names and MAC addresses it has given them to


    Computer networking

    DHCP

    • A DHCP lease will expire after a set amount of time and the computer will have to request a renewal from the DHCP server

    • Expirations are usually 1 day up to 3 months


    Computer networking

    DHCP

    • Why is DHCP useful?

      • Eases network administration -- alternative is to manually configure every computer that connects to the network

      • When a computer moves from one part of a network to another, the IP address info is changed automatically if necessary

      • Prevents possibility that two computers will have an identical IP address (neither would work)


    Routers revisited

    Routers Revisited

    • Routers receive IP datagrams and forward them based on their destination IP addresses


    Routers revisited1

    Routers Revisited

    • Routers can also be used to provide security on a network

    • In the example on the following slide, suppose that we do not want hosts connected to “Switch 2” to be able to open connections to hosts on “Switch 3”


    Routers and security

    Internet

    Routers and Security

    E0: 206.113.116.169

    C

    Switch 1

    E2:

    221.19.10.1

    E1: 220.178.13.2

    E0: 220:178.13.1

    B

    A

    Switch 3

    Switch 2

    E2:

    220.178.17.161

    E1:

    220.178.12.145


    Routers and security1

    Routers and Security

    • This is the same as blocking hosts on the bottom router port E2 from opening connections to hosts on E1


    Routers and security2

    Routers and Security

    • This can be achieved by using “access control lists” – a feature many routers support

    • An example command in a router might look like:

      access 220.178.17.0 220.178.12.0 tcp all deny


    Routers and security3

    Routers and Security

    • Or, suppose that we want to allow hosts on switch 2 (router port E2) to be able to use web servers on switch 1 (router port E1)

    • No other access should be allowed

    • Example commands:

      access 220.178.17.0 220.178.12.0 tcp port 80 allow

      access 220.178.17.0 220.178.12.0 tcp all deny


    Virtual local area network vlan

    Switch

    (Sales)

    Switch

    (R & D)

    Switch

    (Shipping)

    Switch

    (Marketing)

    Switch

    (Management)

    Switch

    (Finance)

    Virtual Local Area Network (VLAN)

    • Suppose:


    Vlans

    VLANs

    • How should this network be secured (using access control lists in the router)?

    • Opinions?


    Vlans1

    VLANs

    • You have your network secure. The Sales people don’t have access to Finance. Marketing doesn’t have access to Engineering, blah blah blah…


    Vlans2

    VLANs

    • Also suppose that you find out they are adding another employee in the Sales department (with a new computer, of course)

    • But there is no room in the Sales department… all the offices are in use

    • So you (the IT manager) get a call saying…


    Vlans3

    VLANs

    • “We are going to put the new Sales guy over in the Finance department area where we have a free office. He’ll need access to all the Sales servers. I assume that isn’t a problem. Thanks. Bye.”


    Vlans4

    VLANs

    • VLANs to the rescue…


    Vlans5

    Switch

    VLANs

    • Simplest example:

    • Six collision domains and one broadcast domain, right?

    • Maybe not, VLANs change that…


    Vlans6

    VLANs

    • If a switch allows VLANs, then the ports on a switch can be divided into groups

    • Each group has a number (a VLAN ID number)

    • Each port on the switch can be assigned to one or more of these VLANs


    Vlans7

    C

    B

    D

    A

    Switch

    E

    F

    VLANs

    • Assign A and B to VLAN 2, all the others to VLAN 3


    Vlans8

    B

    A

    VLANs

    • Results in this “virtual” network:

    C

    D

    Switch

    Switch

    E

    F


    Vlans9

    Switch

    (Sales)

    Switch

    (R & D)

    Switch

    (Shipping)

    Switch

    (Marketing)

    Switch

    (Management)

    Switch

    (Finance)

    VLANs

    • Do we have a solution to our problem yet?


    Vlans10

    VLANs

    • Not yet…

    • We need the router to be a little “smarter”… (i.e. we need the router to be aware of our VLANs)


    Vlans11

    VLANs

    • Time to introduce a new network device (not actually new, just a combination of devices we have talked about already)

    • Mutli-layer Switch

      • Think of it as a combination of a router and a switch


    Multi layer switch

    Multi-layer Switch

    • See sketch on board…


    Vlans12

    Switch

    (Sales)

    Switch

    (R & D)

    Switch

    (Shipping)

    Switch

    (Marketing)

    Switch

    (Management)

    Switch

    (Finance)

    VLANs

    • Now we have a solution…


    Wireless ethernet

    Wireless Ethernet

    • Still Ethernet at layer 2…

    • Wireless is a replacement for layer 1 (UTP, fiber, etc.)


    Wireless ethernet1

    Wireless Ethernet

    • General idea:

    Wireless Access Point


    Wireless ethernet2

    Wireless Ethernet

    • Uses CSMA/CA instead of CSMA/CD

    • CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)

    • Instead of sensing a collision while transmitting and doing a “back off”, hosts must listen to the “network” for a short period of time to determine if it is available


    Wireless ethernet3

    Wireless Ethernet

    • In 1997, IEEE released 802.11 standard for wireless Ethernet

    • The standard was “weak”

    • Companies who made products may have followed the standard, but generally were not compatible with others


    Wireless ethernet4

    Wireless Ethernet

    • In 1999, IEEE released amendments to the 802.11 wireless Ethernet standard:

    • These were 802.11a and 802.11b

    • These amendments provided stronger standards and much greater interoperability between vendors


    802 11a

    802.11a

    • Operates at 5 GHz (gigahertz) radio frequency

    • Maximum transmission rate of 54 Mbit/sec

      • Rate will automatically slow to 48, 36, 24, 18, 12, 9, or 6 Mbit/sec if distance or noise is a problem


    802 11a1

    802.11a

    • Use 12 channels

      • Multiple channels allow multiple access points to operate within the same proximity without interfering with each other

    • Typical range of 100 feet (usually much less – generally only line-of-sight)

    • Widely used but not as popular as…


    802 11b

    802.11b

    • Operates at 2.4 Ghz

    • Maximum transmission rate of 11 Mbit/sec

      • Rate will automatically slow to 5.5 or 2 Mbit/sec if distance or noise is a problem

      • Lucky if you get 5 Mbit/sec

    • Typical range: 100 feet (will go through walls and floors – but not many)


    802 11b1

    802.11b

    • Use 12 channels which overlap

      • Effectively only three channels (1, 6, 11)

      • Only three access points could be located with range of each other

    • Shares frequency with cordless phones and Bluetooth (a problem)


    802 11b2

    802.11b

    • Advantages over 802.11a:

      • Greater range

      • Less expensive equipment

    • Advantages of 802.11a over 802.11b:

      • Faster

      • Less interference from other devices


    802 11g

    802.11g

    • Amendment released in 2003

    • Blend of 802.11a and 802.11b

    • Operates at 2.4 Ghz (like b)

    • Maximum transmission rate of 54 Mbit/s (like a)

      • Rate will automatically adjusts as with 802.11a


    802 11g1

    802.11g

    • Shorter range than 802.11b for maximum data rate, but can go out to 100 feet

    • 802.11g and 802.11b radios could be the same which made upgrading from b to g less expensive (also made dual-mode access points easy to manufacture)


    Next on the horizon 802 11n

    Next on the horizon… 802.11n

    • Amendment not yet ratified

    • Expected release is December 2009

    • Operates at 2.4 GHz or 5.0 GHz

    • If promises hold true,

      • Data rates up to 540 MBit/sec

      • Range up to 165 feet


    Future of wireless ethernet

    Future of Wireless Ethernet?

    • Broadband wireless from cell network providers is a

      • major threat to WiFi

      • major opportunity for consumers


    Next wireless in security

    Next… Wireless (In)security


    Wireless security

    Wireless Security

    • WEP – Wired Equivalent Privacy

    • All users on a wireless AP (and the AP itself) have a shared encryption key

    • The key is used in the process of encrypting and decrypting data being transmitted via the wireless LAN


    Wireless security wep

    Wireless Security - WEP

    • Math – AND, OR, XOR

    • Stream cipher (called the WEP key)

      • composed of 104 bit (13 byte) key + 24 bit “initialization vector”

      • total length 128 bits


    Wireless security wep1

    Wireless Security - WEP

    • To be secure, the stream cipher must never be used more than once

    • The IV is the only thing that is varies on a standard WEP implementation

    • 224 = 16777216 (not many)


    Wireless security wep2

    Wireless Security - WEP

    • 50% chance an IV will repeat after just 5000 frames

    • 99% chance an IV will repeat after 12,000 frames

    • IV is transmitted “in the clear”


    Wireless security wep3

    Wireless Security - WEP

    • Takes less than 2 minutes to break the code (i.e. to figure out the underlying key)

    • WEP as Wired Equivalent Privacy is a joke


    Wpa and wpa2 improvement

    WPA and WPA2 - Improvement

    • WPA – WiFi Protected Access

    • Similar to WEP except: the key changes “on-the-fly” as well as the IV

    • The key is lengthened to 128 bits

    • The IV is 48 bits (harder to snoop)


    Wireless things to remember

    Wireless – Things to Remember

    • Inherently less secure than wired networking

    • Will remain slower than wired, but may become “fast enough” for most users and consumers

    • Major changes in the technology will continue


    Host and domain names

    Host and Domain Names

    • Use of “names” for computers on the Internet eliminates the need for memorizing the IP addresses

    • The naming of computers on the Internet is based on a hierarchical system


    Example of domains and names

    Example of Domains and Names

    • www.erskine.edu

      • EDU – top level domain

      • ERSKINE – our own sub-domain

      • WWW – a host within our domain

    • Other top level domain:

      • Com, org, net, gov, mil, us, biz, info, tv, cc, (each country, page 78)

    • Domain “tree”


    Domain name system

    Domain Name System

    • For two computers to communicate via the Internet, each must know the IP address of the other

    • Domain Name System (DNS)

      • Special servers (nameservers) act as phonebooks for computers on the Internet

    • Certain nameservers have authority for particular domains


    Dns example

    DNS Example

    • My computer

    • Erskine nameserver

    • Root name server

    • www.google.com


    Internet control message protocol icmp

    Internet Control Message Protocol (ICMP)

    • Used by IP for

      • Sending management and control messages between routers and between hosts and routers


    Computer networking

    ICMP

    • Some common ICMP messages:

      • Destination Unreachable

        • If a router can’t send an IP packet any further, it will send a “Destination Unreachable” message back to the originating host

      • Buffer Full

        • If the router is too busy and can’t handle more traffic, it will send out “Buffer Full” messages via ICMP until the traffic jam clears


    Computer networking

    ICMP

    • Datagram Obituary

      • Every IP packet (datagram) has a TTL field in the header

      • The TTL is the number of hops that the packet can take before being discarded

      • If a packet hits the TTL limit, a router will send an ICMP packet back to the originating host informing it of the “execution”


    Icmp utilities

    ICMP Utilities

    • Traceroute (tracert in Windows)

      • Used to determine the path a packet takes as it crosses one or more networks

    • Ping

      • Packet Internet Grouper (I doubt it)

      • Useful for checking layer 2 and layer 3 connectivity between hosts/routers


    Computer networking

    ICMP

    • Hackers used ICMP to cause problems for networks

    • As a result, ICMP now blocked at most network gateways


    Computer networking

    IPv6

    • IP, (IPv4) as we have discussed so far, uses 32 bit (four byte) addresses

    • IPv4 address space = 232 addresses

      • 4,294,967,296 (less than 1 address per person)


    Computer networking

    IPv6

    • IPv6 address space = 2128 addresses

      • Plenty for all personal computers and devices we choose to network

      • 340,282,366,920,938,463,463,374,607,431,768,211,456


    Ipv6 addresses

    IPv6 Addresses

    • Eight four-digit base 16 (hexadecimal) numbers

    • Example:

      • 2001:0db8:85a3:08d3:1319:8a2e:0370:7334


    Ipv6 addresses1

    IPv6 Addresses

    • Network portion (prefix) and host portion

    • 2001:0db8:85a3/48 indicates a subnet with address ranges from:

      2001:0db8:85a3:0000:0000:0000:0000:0000

      to

      2001:0db8:85a3:FFFF:FFFF:FFFF:FFFF:FFFF


    Ipv6 addresses2

    IPv6 Addresses

    • Typically, an IPv6 network will use the first 64 bits for the network and the last 64 bits for the host addresses


    Ipv6 when

    IPv6 – When?

    • When?

      • Mac OS X supports IPv6 now

      • Microsoft Windows Vista supports it now

      • Linux supports it now

      • US Government will require all contractors to use IPv6 by 2008


    Ipv6 when1

    IPv6 – When?

    • Internet “backbone” routers mostly support IPv6 now

    • Most personal computers support IPv6 now

    • Bottleneck is corporate/campus routers


    Ipv6 when2

    IPv6 – When?

    • IPv4 addresses to run out possibly within 4 years or to last as long as 2024

    • Full switch to IPv6 within 10 years -- probably


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