Internet layer

8.   Internet layer

The internet layer has the responsibility of sending packets across potentially multiple networks. Internetworking requires sending data from the source network to the destination network. This process is called routing.

In the Internet protocol suite, the Internet Protocol performs two basic functions:

•    Host addressing and identification: This is accomplished with a hierarchical IP addressing system.

•    Packet routing: This is the basic task of sending packets of data (datagrams) from source to destination by forwarding them to the next network router closer to the final destination.

The internet layer is not only agnostic of application data structures at the transport layer, but it also does not distinguish between operation of the various transport layer protocols. IP can carry data for a variety of different upper layer protocols. These protocols are each identified by a unique protocol number: for example, Internet Control Message Protocol (ICMP) and Internet Group Management Protocol (IGMP) are protocols 1 and 2, respectively.

Some of the protocols carried by IP, such as ICMP which is used to transmit diagnostic information, and IGMP which is used to manage IP Multicast data, are layered on top of IP but perform internetworking functions. This illustrates the differences in the architecture of the TCP/IP stack of the Internet and the OSI model. The TCP/IP model's internet layer corresponds to layer three of the Open Systems Interconnection (OSI) model, where it is referred to as the network layer.

The internet layer provides only an unreliable datagram transmission facility between hosts located on potentially different IP networks by forwarding the transport layer datagrams to an appropriate next-hop router for further relaying to its destination. With this functionality, the internet layer makes possible internetworking, the interworking of different IP networks, and it essentially establishes the Internet. The Internet Protocol is the principal component of the internet layer, and it defines two addressing systems to identify network hosts computers, and to locate them on the network. The original address system of the ARPANET and its successor, the Internet, is Internet Protocol version 4 (IPv4). It uses a 32-bit IP address and is therefore capable of identifying approximately four billion hosts. This limitation was eliminated by the standardization of Internet Protocol version 6 (IPv6) in 1998, and beginning production implementations in approximately 2006.