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The Network Layer in

the Internet

Overview

^ Internet Intro
^ The IP Protocol
^ IP Addresses
^ Subnets
^ Internet Control Protocols
^ CIDR – Classless InterDomain Routing
^ IPv

Internet Intro

^ A sketch of this semi-hierarchicalorganization is given below:

Internet Intro

^ The glue that holds the Internet togetheris the network layer protocol, IP (InternetProtocol).
^ Unlike most older network layer protocols,it was designed from the beginning withinternetworking in mind.
^ A good way to think of the network layeris this: its job is to provide a best-effortway to transport datagrams (packets)from source to destination, without regardto whether or not these machines are onthe same network, or whether or notthere are other networks in the betweenthem.

Internet Intro

^ Communication in the Internet works asfollows:^ • The transport layer takes data streams andbreaks them up into datagrams.• In theory datagrams can be up to 64KB each,but in practice they are usually around1500bytes. Each datagram is transmittedthrough the Internet, possibly beingfragmented into smaller units as it goes.• When all the pieces finally get to thedestination machine, they are reassembled bythe network layer into the original datagram.• This datagram is then handed to the transportlayer, which inserts it into the receivingprocess’ input stream.

The IP Protocol

^ An appropriate place to start our study ofthe network layer in the Internet is theformat of the IP datagrams themselves.
^ An^ IP datagram

consists of:

• a^ header

part and

• a^ data/text

part
^ The header has a 20-byte fixed part and avariable length optional part.

10

The IP Protocol

^ The^

Version

field keeps track of which version of the protocol the datagrambelongs to.
Since the header length is not constant, afield in the header, called

IHL, is provided

to tell how long the

header is, in 32 bit

words. (min 5, max 15)
The^ type of service

field allows the host to

tell the subnet what kind of service itwants. (various combinations of reliabilityand speed are possible)
The^ total length

includes everything in the datagram – both header and data. Themaximum length is 65,535 bytes.