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COS 420 . Day 3. Agenda. Assignment Due Jan 29, 2003 Next Class Individual Projects assigned Today. Individualized project. Will be a research project & paper ~ 20 page paper MLA Format 10 Min Presentation You can pick any topic that one of the IETF working groups is developing

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Cos 420

COS 420

Day 3


  • Assignment Due Jan 29, 2003

    • Next Class

  • Individual Projects assigned Today

Individualized project
Individualized project

  • Will be a research project & paper

    • ~ 20 page paper

      • MLA Format

    • 10 Min Presentation

  • You can pick any topic that one of the IETF working groups is developing


    • Past history, Current state, upcoming developments

  • Due Date

    • Papers due March 15

    • Presentations will be on March 22

Classful internet addresses

  • Definitions

    • Name

      • Identifies what an entity is

      • Often textual (e.g., ASCII)

    • Address

      • Identifies where an entity is located

      • Often binary and usually compact

      • Sometimes called locator

    • Route

      • Identifies how to get to the object

      • May be distributed

Internet protocol address ip address
Internet Protocol Address(IP Address)

  • Analogous to hardware address

  • Unique value assigned as unicast address to each host on Internet

  • Used by Internet applications

Ip address details
IP Address Details

  • 32-bit binary value

  • Unique value assigned to each host in Internet

  • Values chosen to make routing efficient

Ip address division
IP Address Division

  • Address divided into two parts

    • Prefix (network ID) identifies network to which host attaches

    • Suffix (host ID) identifies host on that network

Classful addressing
Classful Addressing

  • Original IP scheme

  • Explains many design decisions

  • New schemes are backward compatible

Desirable properties of an internet addressing scheme
Desirable Properties Of AnInternet Addressing Scheme

  • Compact (as small as possible)

  • Universal (big enough)

  • Works with all network hardware

  • Supports efficient decision making

    • Test whether a destination can be reached directly

    • Decide which router to use for indirect delivery

    • Choose next router along a path to the destination

Division of internet address into prefix and suffix
Division Of Internet AddressInto Prefix And Suffix

  • How should division be made?

    • Large prefix, small suffix means many possible networks, but each is limited in size

    • Large suffix, small prefix means each network can be large, but there can only be a few networks

  • Original Internet address scheme designed to accommodate both possibilities

    • Known as classful addressing

Important property
Important Property

  • Classful addresses are self-identifying

  • Consequences

    • Can determine boundary between prefix and suffix from the address itself

    • No additional state needed to store boundary information

    • Both hosts and routers benefit

Endpoint identification
Endpoint Identification

  • Because IP addresses encode both a network and a host on that network, they do not specify an individual computer, but a connection to a network.

Ip address conventions
IP Address Conventions

  • When used to refer to a network

    • Host field contains all 0 bits

  • Broadcast on the local wire

    • Network and host fields both contain all 1 bits

  • Directed broadcast: broadcast on specific (possibly remote) network

    • Host field contains all 1 bits

    • Nonstandard form: host field contains all 0 bits

Assignment of ip addresses
Assignment Of IP Addresses

  • All hosts on same network assigned same address prefix

    • Prefixes assigned by central authority

    • Obtained from ISP

  • Each host on a network has a unique suffix

    • Assigned locally

    • Local administrator must ensure uniqueness

Advantages of classful addressing
Advantages Of Classful Addressing

  • Computationally efficient

    • First bits specify size of prefix / suffix

  • Allows mixtures of large and small networks

Directed broadcast
Directed Broadcast

  • IP addresses can be used to specify a directed broadcast in which a packet is sent to all computers on a network; such addresses map to hardware broadcast, if available. By convention, a directed broadcast address has a valid netid and has a hostid with all bits set to 1.

Limited broadcast
Limited Broadcast

  • All 1’s

  • Broadcast limited to local network only (no forwarding)

  • Useful for bootstrapping

All zeros ip address
All Zeros IP Address

  • Can only appear as source address

  • Used during bootstrap before computer knows its address

  • Means ‘‘this’’ computer

Internet multicast
Internet Multicast

  • IP allows Internet multicast, but no Internet-wide multicast delivery system currently in place

  • Class D addresses reserved for multicast

  • Each address corresponds to group of participating computers

  • IP multicast uses hardware multicast when available

  • More later in the course

Consequences of ip addressing
Consequences Of IP Addressing

  • If a host computer moves from one network to another, its IP address must change

  • For a multi-homed host (with two or more addresses), the path taken by packets depends on the address used

Multi homed hosts and reliability
Multi-Homed Hosts And Reliability

  • Knowing that B is multi-homed increases reliability

Dotted decimal notation
Dotted Decimal Notation

  • Syntactic form for expressing 32-bit address

  • Used throughout the Internet and associated literature

  • Represents each octet in decimal separated by periods (dots)

Example of dotted decimal notation
Example Of Dotted DecimalNotation

  • A 32-bit number in binary

    • 10000000 00001010 00000010 00000011

  • The same 32-bit number expressed in dotted decimal notation

    • 128 . 10 . 2 . 3

Loopback address
Loopback Address

  • Used for testing

  • Refers to local computer (never sent to Internet)

  • Address is

Example of ip addressing
Example Of IP Addressing

  • Assume an organization has three networks

  • Organization obtains three prefixes, one per network

  • Host address must begin with network prefix


  • IP address

    • 32 bits long

    • Prefix identifies network

    • Suffix identifies host

  • Classful addressing uses first few bits of address to determine boundary between prefix and suffix Special forms of addresses handle

    • Limited broadcast

    • Directed broadcast

    • Network identification

    • This host

    • Loopback

Part v






  • Must use hardware (physical) addresses to communicate over network

  • Applications only use Internet addresses


  • Computers A and B on same network

  • Application on A generates packet for application on B

  • Protocol software on A must use B’s hardware address when sending a packet


  • Protocol software needs a mechanism that maps an IP address to equivalent hardware address

  • Known as address resolution problem

Address resolution
Address Resolution

  • Performed at each step along path through Internet

  • Two basic algorithms

    • Direct mapping

    • Dynamic binding

  • Choice depends on type of hardware

Direct mapping
Direct Mapping

  • Easy to understand

  • Efficient

  • Only works when hardware address is small

  • Technique: assign computer an IP address that encodes the hardware address

Example of direct mapping
Example Of Direct Mapping

  • Hardware: proNet ring network

  • Hardware address: 8 bits

  • Assume IP address (24-bit prefix)

  • Assign computer with hardware address K an IP address

  • 192.5.48.K

  • Resolving an IP address means extracting the hardware address from low-order 8 bits

Dynamic binding
Dynamic Binding

  • Needed when hardware addresses are large (e.g., Ethernet)

  • Allows computer A to find computer B’s hardware address

    • A starts with B’s IP address

    • A knows B is on the local network

  • Technique: broadcast query and obtain response

    Note: dynamic binding only used across one network at a time

Internet address resolution protocol arp
Internet Address Resolution Protocol (ARP)

  • Standard for dynamic address resolution in the Internet

  • Requires hardware broadcast

  • Intended for LAN

  • Important idea: ARP only used to map addresses within a single physical network, never across multiple networks


  • Machine A broadcasts ARP request with B’s IP address

  • All machines on local net receive broadcast

  • Machine B replies with its physical address

  • Machine A adds B’s address information to its table

  • Machine A delivers packet directly to B

Illustration of arp request and reply messages
Illustration Of ARPRequest And Reply Messages

Arp packet format when used with ethernet
ARP Packet Format WhenUsed With Ethernet

Observations about packet format
Observations About Packet Format

  • General: can be used with

    • Arbitrary hardware address

    • Arbitrary protocol address (not just IP)

  • Variable length fields (depends on type of addresses)

  • Length fields allow parsing of packet by computer that does not understand the two address types

Retention of bindings
Retention Of Bindings

  • Cannot afford to send ARP request for each packet

  • Solution

    • Maintain a table of bindings

  • Effect

    • Use ARP one time, place results in table, and then send many packets

Arp caching
ARP Caching

  • ARP table is a cache

  • Entries time out and are removed

  • Avoids stale bindings

  • Typical timeout: 20 minutes

Algorithm for processing arp requests
Algorithm For ProcessingARP Requests

  • Extract sender’s pair, (IA, EA) and update local ARP table if it exists

  • If this is a request and the target is ‘‘me’’

    • Add sender’s pair to ARP table if not present

    • Fill in target hardware address

    • Exchange sender and target entries

    • Set operation to reply

    • Send reply back to requester

Algorithm features
Algorithm Features

  • If A ARPs B, B keeps A’s information

    • B will probably send a packet to A soon

  • If A ARPs B, other machines do not keep A’s information

    • Avoids clogging ARP caches needlessly

Conceptual purpose of arp
Conceptual Purpose Of ARP

  • Isolates hardware address at low level

  • Allows application programs to use IP addresses

Arp encapsulation
ARP Encapsulation

  • ARP message travels in data portion of network frame

  • We say ARP message is encapsulated

Ethernet encapsulation
Ethernet Encapsulation

  • ARP message placed in frame data area

  • Data area padded with zeroes if ARP message is shorter

  • than minimum Ethernet frame

  • Ethernet type 0x0806 used for ARP


  • Computer’s IP address independent of computer’s hardware address

  • Applications use IP addresses

  • Hardware only understands hardware addresses

  • Must map from IP address to hardware address fortransmission

  • Two types

    • Direct mapping

    • Dynamic mapping

  • Address Resolution Protocol (ARP) used for dynamic address mapping

  • Important for Ethernet

  • Sender broadcasts ARP request, and target sends ARP reply

  • ARP bindings are cached

Part vi




Ip address assignment
IP Address Assignment

  • For conventional computer

    • IP address stored on disk

    • OS obtains address by reading from file at startup

  • For diskless computer

    • IP address obtained from server

Reverse address resolution protocol rarp
Reverse Address ResolutionProtocol (RARP)

  • Old protocol

  • Designed for diskless computer

  • Obtains an IP address

  • Adapted from ARP

  • Broadcasts request to server

  • Waits for response

Ethernet encapsulation1
Ethernet Encapsulation

  • RARP message carried in data portion of Ethernet frame

  • Ethernet type 0x0835 assigned to RARP

Illustration of packet flow
Illustration Of Packet Flow

  • In (a) client broadcasts a request

  • In (b) one or more servers respond

Client identification
Client Identification

  • Computer must identify itself

  • RARP uses network hardware address as unique ID

    • Only works on network with permanent address (e.g., Ethernet)

Modern bootstrap
Modern Bootstrap

  • Except for a few special cases, RARP has largely been replaced by DHCP

  • We will postpone further discussion of bootstrapping until later in the course when we can consider DHCP

For next week
For next week

  • Assignment #2 will be posted

  • We begin a more in depth look at IP

    • IP Architecture

    • IP routing

    • IP error and control messaging