The Internetwork Protocol (IP)

The IP (Internet Protocol) is a protocol that uses datagrams to communicate over a packet-switched network, such as the Internet. The IP protocol operates at the network layer protocol of the OSI reference model and is a part of a suite of protocols known as TCP/IP. Today, with over 1.5 billion users worldwide, the current Internet is a great success in terms of connecting people and communities. Even though the current Internet continues to work and is capable of fulfilling its current missions, it also suffers from a relative “ossification”, a condition where technological innovation meets natural resistance, as exemplified by the current lack of wide deployment of technologies such as multicast or Internet Protocol version 6 (IPv6).

The Internetwork Protocol (IP) [RFC791] provides a best effort network layer service connecting endpoints (computers, phones, etc) to form a computer network. In IPv4, each endpoint is identified by one or more gloablly unique IP addresses. The network layer PDUs are known as either "packets" or "datagrams". Each packet carries the source IP address of the sending endpoint and also the address of the intended recipient endpoint (or a group destination address). Other protocol information is also carried.

The IP network service transmits datagrams between routers (intermediate nodes) using IP routers. The routers themselves can be simple, since no information is stored concerning the datagrams that are forwarded on a link. The most complex part of an IP router is concerned with determining the optimum link to use to reach each destination in a network. This process is known as "routing". Although the routing process is computationally intensive, it is only performed at periodic intervals.

An IP network normally uses a dynamic routing protocol to find alternate routes whenever a link becomes unavailable. This provides considerable robustness from the failure of either links or routers, but is unable to guarentee reliable delivery. Some applications are happy with this basic service and use a simple transport protocol known as the User Datagram Protocol (UDP) to access this best effort service.

Most Internet users need additional functions such as end-to-end error and sequence control to give a reliable service (equivalent to that provided by virtual circuits). This reliability is usually provided by the Transmission Control Protocol (TCP), which is used end-to-end across the Internet.

In a LAN environment, the protocol is normally carried by Ethernet, but for long distance links, other link protocols using fibre optic links are usually used. Other protocols associated with the IPv4 network layer are the Internet Control Message Protocol (ICMP) and the Address Resolution Protocol (arp).


The IPv4 protocol although widely used, is slowly being superceded by IPv6 [RFC2460], a next-generation network-layer protocol. IPv6 is now widely implemented, and deployed in many networks.

The transition from IPv4 towards a majority IPv6 deployment will take many years to complete, and indeed IPv4 may never itself be phased out completely. In the meantime the two protocols can co-exist and be used together in various ways. Most people think that IPv6 will ultimately become the dominant version of IP used in the Internet. IPv6 changes many things, one of the most obvious from the Ethernet perspective is that it uses a different Ether-Types and uses the Neighbor-Discovery (ND) protocol in place of ARP.

Why name the next version after IPv4 as IPv6?

The new protocol is IPv6. (The version number "5" had already been used for an experimental protocol, called ST-2, which has not stood the test of time.

Why name the next version after IPv4 as IPv6?

The new protocol is IPv6. (The version number "5" had already been used for an experimental protocol, called ST-2, which has not stood the test of time.

See also:

IP Packet Header

IP Packet Processing

Multi-Protocol Label Switching (MPLS)


J. Postel Internet Protocol (STD0005) RFC791

Baker, F., Ed., "Requirements for IP Version 4 Routers", RFC 1812, June 1995.

Hornig, C., "A Standard for the Transmission of IP Datagrams over Ethernet Networks", STD 41, RFC 894, April 1 1984.

Plummer, D., "Ethernet Address Resolution Protocol: Or Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware", STD 37, RFC 826, November 1982.

Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.

Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, December 1998.

Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.

Gorry Fairhurst - - Date: 19/06/2017 EG3567