Network Layer of the OSI Model

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.

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.