IP header

An IP header is

) 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 is the fourth version in the development of the Internet Protocol, and routes most traffic on the Internet.^{[1][non-primary source needed]} 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.

. The header in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers.