An IP header is
header information at the beginning of an
Internet Protocol (IP) packet. An IP packet is the smallest message entity exchanged via the Internet Protocol across an
IP network. IP packets consist of a header for addressing and routing, and a
payload for user data. The header contains information about IP version, source
IP address
An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
, destination IP address,
time-to-live, etc. The payload of an IP packet is typically a
datagram
A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The del ...
or segment of the higher-level
transport layer protocol, but may be data for an
internet layer (e.g.,
ICMP or
ICMPv6) or
link layer (e.g.,
OSPF) instead.
Two different versions of IP are used in practice today: IPv4 and IPv6. The IPv6 header uses
IPv6 addresses and thus offers a much bigger
address space but is not backward compatible with IPv4.
IPv4
IPv4 is the fourth version in the development of the
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
, and routes most traffic on the
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...
.
The IPv4 header includes thirteen mandatory fields and is as small as 20 bytes. A fourteenth optional and infrequently used ''options'' field can increase the header size.
IPv6
IPv6
Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...
is the successor to IPv4 and has a different header layout. It was defined in 1998 and is in various stages of
production deployment. The header in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers.
References
{{reflist
Internet Protocol