Network Layer of the OSI Model

Routed Protocols

Routed protocols are those whose addressing schemes differentiate between a network and host portion, allowing a host and its network to be uniquely identified. The main responsibility of a routed protocol is moving data between hosts, across networks. Examples of routed protocols include IP, IPX, and AppleTalk. Consider the IP, IPX, and AppleTalk addresses below. Notice the split between the network and host portion of the address in each case.

Examples of IP, IPX, and AppleTalk addresses: (IP): IP addresses are represented in dotted-decimal notation. In this case, 149.234 represents the network, while 16.3 represents the host.

75E3F210.001022EA27BE (IPX): IPX addresses are represented in hexadecimal. The first 32 bits (75E3F210) represent the network; the final 48 bits (001022EA27BE) represent the host.

1578.6 (AppleTalk): AppleTalk addresses are represented in dotted-decimal notation. The network number is a 16-bit number, in this case ‘1578’. The host is represented by an 8-bit node number, in this case ‘6’

Routing Protocols

Routing protocols are those used by routers to exchange information stored in their routing tables. For example, if you wanted three routers on your network to exchange information with one another dynamically, you would configure each with a common routing protocol. This would allow them to ‘talk’ to each other, exchanging information about the networks they know about. Common routing protocols that you’ll find on networks include the Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Interior Gateway Routing Protocol (IGRP). These routing protocols and others will be discussed in detail in Chapter 8.

Tip: Routers exist at the Network Layer of the OSI model.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.