Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 20
IP Overview

The Internet Protocol (IP) is situated at the network layer of the OSI model and is designed to interconnect packet switched communication networks to form an internet. It transmits blocks of data called datagrams received from the IP’s upper–layer software to and from source and destination hosts. It provides a best effort or connectionless delivery service between the source and destination—connectionless in that it does not establish a session between the source and destination before it transmits its data. This is the layer that is also responsible for the IP protocol addressing.

In order to allow for multiple IP networks to interoperate, there must be a mechanism to provide flow between the differently addressed systems. The device that routes data between different IP addressed networks is called a router, which is often erroneously thought of as being the only function of the IP layer. It is not, and this is explained in more detail later. The router is basically a traffic cop. You tell the traffic cop where you want to go and he points you in the right direction. Routers contain ports that are physical connections to networks. Each of these ports must be assigned a local address. With more than one router, each router must know the others’ configured information. We could configure all the IP addresses and their associated ports on a router statically, but this is a very time–consuming and nonefficient method. Therefore, we have protocols that distribute the IP address information to each router. These are called routing protocols. The two main types for IP networks are RIP (Routing Information Protocol, version 1 or 2) and OSPF (Open Shortest Path First). Both are known as Interior Gateway Protocols (IGPs), protocols that run within a single autonomous systems. An autonomous system is a collection of networks and routers that is under one administrative domain. For example, if you work for the Timbuktu Company and you have seven regional offices in the United States, all communication between those offices is accomplished via routers all running RIP. You have one domain known as Timbuktu.com; therefore, all the networks and routers and computer equipment is under one administrative domain. Connection to the outside world via the Internet (which is another domain) allows communication with another company that is under another administrative domain.

You should be aware there are two version of IP: IPv4 (version 4, the current IP) and IPv6 (version 6, the experimental IP). IPv4 continues to operate admirably, but has become strained with “patches” to make it continue to work. The latest is the address scheme and IPv6 was partially motivated by the inability to scale and the exhaustion of IP Class B addresses. IPv6 is a natural evolution of IP and extends the address space to 128 bits and cleans up a lot of unused functions.