The internetwork layer, also called the internet layer or the network layer, provides the "virtual network" image of an internet. This layer shields the higher levels from the physical network architecture below it.

Internet Protocol (IP) is the most important protocol in this layer. It is a connectionless protocol that doesn't assume reliability from lower layers. IP does not provide reliability, flow control, or error recovery. These functions must be provided at a higher level.
IP provides a routing function that attempts to deliver transmitted messages to their destination.

A message unit in an IP network is called an IP datagram. This is the basic unit of information transmitted across TCP/IP networks.

Other internetwork layer protocols are IP, ICMP, IGMP, ARP and RARP.