Network Layer of the OSI Model

By Dan DiNicolo, August 25th, 2005 Posted in CCNA Study Guide Chapter 01. Subscribe to our RSS Feed

The Network layer of the OSI model is commonly referred to as Layer 3, and has the following responsibilities:

• Routing. When a host on one network wishes to exchange data with a host on another, packets will be sent to a router interface. After determining where the packet should be forwarded next using information found in its routing table, a router will switch the packets out of the optimal interface. This process will take place at each router encountered on a packet’s journey to the destination host. Routing protocols are used to allow routers to exchange information with one another.
• Network Addressing. Each host on a routed internetwork will have at least one network address. A network address is made up of two parts – the first part identifies the network, while the second identifies a unique host on that network. These addresses have different formats depending on the routed protocol in use – we’ll look at examples shortly.

Examples of Network-layer protocols:

Internet Protocol (IP): TCP/IP addressing and routing

Internetwork Packet Exchange (IPX): IPX/SPX addressing and routing

Internet Control Message Protocol (ICMP): Diagnostics and error notification

Internet Group Management Protocol (IGMP): Multicast group management

Tip: Remember that the protocol data unit (PDU) of the Network layer is referred to as a packet. In some cases, you may also see this PDU referred to as a datagram.

Written by Dan DiNicolo - Visit Website

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