Table of Contents 
SummaryOne of the basic functions of IP is its ability to form connections between different physical networks. This is due to the flexibility of IP to use almost any physical network below it, and to the IP routing algorithm. A system which does this is termed a router, although the older term IP gateway is also used.
Note: In other sections of the book, we show the position of each protocol in the layered model of the TCP/IP protocol stack. The routing function is part of the internetwork layer, but the primary function of a routing protocol is to exchange routing information with other routers, and in this respect the protocols behave more like application protocols. The routing protocols described here use all three approaches to data transport: using UDP (for example RIP, described in Routing Information Protocol (RIP)), TCP (see BGP in Border Gateway Protocol (BGP)) and providing its own transport layer on top of IP (see OSPF in Open Shortest Path First Protocol (OSPF) Version 2). Therefore, we shall not attempt to represent the position of these protocols in the protocol stack with a diagram as we do with the other protocols.
Basic
IP Routing