(Transmission Control Protocol/Internet Protocol) A communications protocol developed under contract from the U.S. Department of Defense to internetwork dissimilar systems. Invented by Vinton Cerf and Bob Kahn, this de facto Unix standard is the protocol of the Internet and has become the global standard for communications.

TCP provides transport functions, which ensures that the total amount of bytes sent is received correctly at the other end. UDP, which is part of the TCP/IP suite, is an alternate transport that does not guarantee delivery. It is widely used for realtime voice and video transmissions where erroneous packets are not retransmitted.

TCP/IP is a routable protocol, and the IP part of TCP/IP provides this capability. In a routable protocol, all messages contain not only the address of the destination station, but the address of a destination network. This allows TCP/IP messages to be sent to multiple networks (subnets) within an organization or around the world, hence its use in the worldwide Internet.

The IP network layer (layer 3) of the TCP/IP protocol stack accepts packets from the TCP or UDP transport layer (layer 4), adds its own header and delivers a "datagram" to the data link layer protocol (layer 2).

Every client and server in a TCP/IP network requires an IP address, which is either permanently assigned or dynamically assigned at startup.

For an explanation of how the various layers in TCP/IP work, see TCP/IP abc's and TCP/IP port. See also IP address, NFS, NIS, DNS, DHCP, dumb network and IP on Everything.
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