An IP header is header information at the beginning of an IP packet which contains information about IP version, source IP address, destination IP address, time-to-live, etc.
Two different versions of IP headers have been defined, IPv4 and IPv6. The IPv6 header uses IPv6 addresses and thus offers a much bigger address space, but is not backwards compatible with IPv4.
IP header Wikipedia
IPv4 is the fourth version in the development of the Internet Protocol (IP), and routes most traffic on the Internet. IPv4 is described in IETF publication RFC 791 (September 1981), replacing an earlier definition (RFC 760, January 1980).
IPv4 is a connectionless protocol for use on packet-switched networks. It operates on a best effort delivery model, in that it does not guarantee delivery, nor does it assure proper sequencing or avoidance of duplicate delivery. These aspects, including data integrity, are addressed by an upper layer transport protocol, such as the Transmission Control Protocol (TCP).
IPv6, the successor to IPv4, has been defined and is in various stages of production deployment, and has a different header layout.
An IPv6 packet is the smallest message entity exchanged via the Internet Protocol across an IPv6 network. Packets consist of control information for addressing and routing, and a payload consisting of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level Transport Layer protocol, but may be data for an Internet Layer (e.g., ICMPv6) or Link Layer (e.g., OSPF) instead.