Internet Protocol (IP)

Definition of Internet Protocol (IP) in The Network Encyclopedia.

What is IP (Internet Protocol)?

A TCP/IP network layer protocol for addressing and routing packets of data between hosts on a TCP/IP network. Internet Protocol (IP) is a connectionless protocol that provides best-effort delivery using packet-switching services.

How it works

IP does not guarantee delivery of data. The responsibility for guaranteeing delivery and sending acknowledgments lies with the higher transport-level protocol Transmission Control Protocol (TCP).

The structure of an IP packet is shown in the following diagram. Some of the more important header fields include

  • Source IP address:
    The IP address of the host transmitting the packet.


  • Destination IP address:
    The IP address of the host to which the packet is being sent, a multicast group address, or the broadcast IP address


  • Header checksum:
    A mathematical computation used for verifying that the packet was received intact.


  • Time to Live (TTL):
    The number of router hops that the packet can make before being discarded.


  • Fragment offset:
    The position of the fragment if the original IP packet has been fragmented (for example, by a router). This information enables the original packet to be reconstructed.


IP packets are routed in the following fashion:

  • If IP determines that the destination IP address is a local address, it transmits the packet directly to the destination host.
  • If IP determines that the destination IP address is a remote address, it examines the local routing table for a route to the destination host. If a route is found, it is used; if no route is found, IP forwards the packet to the default gateway. In either case, the packet destined for a remote address is usually sent to a router.
  • At the router, the TTL is decreased by 1 or more (depending on network congestion), and the packet might be fragmented into smaller packets if necessary. The router then determines whether to forward the packet to one of the router’s local network interfaces or to another router. This process repeats until the packet arrives at the destination host or has its TTL decremented to 0 (zero) and is discarded by a router.

    Graphic I-11. The structure of an IP packet.