ISO's OSI Layered Model - Network Layer

The way that the data will be sent to the recipient device is determined in this layer. The famous Internet Protocol (IP) resides in this layer. The Internetwork Protocal identifies each host with a 32-bit IP address (for detail, see understanding subnet mask). IP is responsible for routing, directing datagrams from one network to another. It manages the connectionless transfer of data one hop at a time, from router to router. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. It is like a postal department, where the letter is passed from location to location, until it reaches the destination address on the envelope. The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram.

Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The DataLink layer protocal Address Resolution Protocol (ARP) is used to map the IP address to its hardware address.

In summary, the main functionality of network layer is routing and logical addressing, the data unit is packets, the network layer device is router and the network layer examples include IP, IPX, IPsec, ICMP, IGMP, OSPF, IGRP and EIGRP.

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