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CSCI 233 Internet Protocols Class 3 - PowerPoint PPT Presentation

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CSCI 233 Internet Protocols Class 3. David C. Roberts. First…a little review. Internet Protocol Principles . Good Citizen Principle Scarcest Internet Resource. Outline. Internet addresses Mapping IP addresses to physical addresses. Internet addressing. The Internet.

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CSCI 233Internet Protocols

Class 3

David C. Roberts

Internet protocol principles
Internet Protocol Principles

  • Good Citizen Principle

  • Scarcest Internet Resource


  • Internet addresses

  • Mapping IP addresses to physical addresses

The internet
The Internet

  • Is it a physical or virtual network?

  • It’s a virtual network, defined by protocols that run on hosts and routers.

  • Internet protocols make the Internet look like a world-wide uniform network, although it encompasses many networks that are very different from each other.

Internet addresses
Internet Addresses

  • Each host connection on the Internet has a unique Internet address

  • The addresses are designed to make forwarding of Internet packets simple

  • An IP address has two parts: a prefix that identifies a network and a suffix that identifies a host on the network

Need for control
Need for Control

  • To avoid conflicts in address use, some sort of authority is needed

  • It makes sense to assign addresses in blocks, not one at a time

  • ICANN (Internet Corporation for Assigned names and Numbers) oversees IP address assignment

  • Originally assigned in blocks of Class A, B and C addresses


  • How many Internet addresses can one host have?

  • As many as it has network adapters

Ipv6 addressing
IPv6 Addressing

  • Each address is 128 bits

  • Enough addresses for every person on earth to have an internet with three times the addresses of the present Internet!

  • 1024 addresses per square meter of the earth’s surface

Functions of internet addresses
Functions of Internet Addresses

  • Provide a unique identification for a particular interface between a device and the network so that a datagram can be delivered to the correct recipient

  • Enable a path to be found across the Internet to reach the recipient, a process called routing

Ipv4 address
IPv4 Address

  • 32-bit integer, unique for each host on the network, used in all communication with the host

  • <IP address> ::= <netid> <hostid>

    • Netid: identifier of a network

    • Hostid: identifier of a host on the network

Dotted decimal notation
Dotted Decimal Notation

32-bit Internet address

10000000 00001010 00000010 00011110

Is written

Classes of ipv4 addresses
Classes of IPv4 Addresses

“Classful” addresses—types A, B and C below

first 2 bits distinguish 3 primary classes

Design of these classes is for efficient routing

There have been other refinements—to discuss later

Advantages of classful addressing
Advantages of Classful Addressing

  • Simplicity and clarity—addresses and their setup are very easy to understand

  • Flexibility to accommodate different sizes of networks

  • Ease of separating host address for routing

  • Allows for reservation of some addresses for special purposes

Classless ipv4 addressing
Classless IPv4 Addressing

  • Temporary addressing scheme that does away with class A, B, C addresses

  • Network prefix can be any specified length

  • Forwarding techniques expanded to account for this: called Classless Inter-Domain Routing (CIDR)

Ip and mac addresses
IP and MAC Addresses

  • IP address is used to send datagrams across the Internet—that is, between networks, through routers

  • MAC address is used to deliver a frame of data within a single network

  • We send a datagram across the Internet with only an IP address

  • To deliver to a device at the destination network, a MAC address must be used

Local delivery
Local Delivery

  • The router at the destination network has the job of delivering the packet to the appropriate host

  • The router uses the local physical network to deliver to the local host

  • The local physical (MAC) address must be used, not the IP address

Address resolution1
Address Resolution

  • The process of determining the physical address that corresponds to an IP address is called address resolution

  • Address resolution must occur at every network the packet encounters in its journey across the Internet

Resolution by dynamic binding
Resolution by Dynamic Binding

  • ARP broadcasts a request packet

  • Host who has IP address in packet replies with physical address

Arp caching
ARP Caching

  • Broadcasting an inquiry is expensive

  • Every host must have a cache of recently acquired bindings

  • Results of ARP requests are cached

  • Before sending request, the cache is checked

Arp cache timeout
ARP Cache Timeout

  • Responsibility for cache correctness is with the host maintaining the cache

  • Timeout value is set, and addresses from cache are not used if timeout value is exceeded

  • ARP performance is sensitive to the value of the timeout

Soft state
Soft State

  • ARP cache is an example of “soft state”

  • Cache owner keeps record of acquired results, avoids cost of future inquiries

  • Cache is usually timed out to automatically remove stale values

Arp refinements
ARP Refinements

  • Every ARP request has binding of source IP and MAC addresses

  • Since request is broadcast, all machines can extract sender’s IP to MAC address mapping and cache it

  • Most computers broadcast a gratuitous ARP request when they start up in case their mapping has changed

Ipv6 arp

  • Describe IPv6 ARP

  • There isn’t any!!!

Ipv4 arp message format
IPv4 ARP Message Format

Hardware type: 1 for Ethernet

Protocol type: 0800 for IP addresses

Xlen—length of physical and high-level addresses

ARP exchanges involve filling in missing addresses

Ipv6 neighbor discovery
IPv6 Neighbor Discovery

  • Neighbor is another computer on the same network

  • NDP allows an IPv6 host to discover all neighbors and routers upon startup

  • Early binding avoids delays when packets are transmitted


  • 1’s refer to “all”

  • 0’s refer to “this”

  • Hostid=0 address refers to this network

  • Hostid=1’s broadcast to all hosts

    • Directed broadcast—netid of a specific network

    • Local broadcast—32 1’s—used at startup

Address resolution2
Address Resolution

  • Physical addresses are used by hardware devices that can communicate directly (ie, MAC addresses on a LAN)

  • IP addresses create a massive virtual network

  • Network layer sends datagrams across the virtual network

  • Data link layer sends frames between physical devices


  • Each host has an Internet address, Ia

  • Each host also has a physical address, Pa

  • How to route packet to physical address, given its Internet address?

  • Two instances

    • Sending packets to routers, which have physical addresses

    • Sending packets to hosts, which have physical addresses

Direct mapping resolution
Direct Mapping Resolution

  • Choose a numbering scheme that makes address resolution efficient

    PA = f(IA)

  • If either P’s or I’s can be chosen, a correspondence can be established

  • Alternatively, lists of P-A pairs can be stored

Dynamic binding resolution
Dynamic Binding Resolution

ARP—Address Resolution Protocol

  • host A broadcasts packet with address IB

  • Asks host B to respond with PB

  • B recognizes the packet, responds with PB

  • A receives response, uses PB to send to B

Arp cache
ARP Cache

  • Cache of recently-acquired physical addresses is kept

  • ARP is used for packets after the first in a transmission

  • ARP cache times out after an interval

  • Example of “soft state”

Arp refinements1
ARP Refinements

  • Sender includes its own I to P binding in every ARP broadcast, so that receiving site, and others, can update ARP caches

  • Receivers update I to P binding in ARP cache before processing ARP packet

Internet protocol principles1
Internet Protocol Principles

  • Use the Good Citizen Principle to limit the impact of resource shortages

  • Conserve the scarcest Internet resource

  • Use caching to avoid repeated inquiries


  • IP Addressing

  • IP Address Resolution