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.