TCP/IP

TCP/IP is a two-layer program. The higher layer, Transmission Control Protocol, manages the assembling of a message or file into smaller packets that are transmitted over the Internet and received by a TCP layer that reassembles the packets into the original message. The lower layer, Internet Protocol, handles the address part of each packet so that it gets to the right destination. Each gateway computer on the network checks this address to see where to forward the message. Even though some packets from the same message are routed differently than others, they'll be reassembled at the destination.

TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided a service (such as sending a Web page) by another computer (a server) in the network. TCP/IP communication is primarily point-to-point, meaning each communication is from one point (or host computer) in the network to another point or host computer. TCP/IP and the higher-level applications that use it are collectively said to be "stateless" because each client request is considered a new request unrelated to any previous one (unlike ordinary phone conversations that require a dedicated connection for the call duration). Being stateless frees network paths so that everyone can use them continuously. (Note that the TCP layer itself is not stateless as far as any one message is concerned. Its connection remains in place until all packets in a message have been received.)

Many Internet users are familiar with the even higher layer application protocols that use TCP/IP to get to the Internet. These include the World Wide Web's Hypertext Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet (Telnet) which lets you logon to remote computers, and the Simple Mail Transfer Protocol (SMTP). These and other protocols are often packaged together with TCP/IP as a "suite."

Personal computer users with an analog phone modem connection to the Internet usually get to the Internet through the Serial Line Internet Protocol (SLIP) or the Point-to-Point Protocol (PPP). These protocols encapsulate the IP packets so that they can be sent over the dial-up phone connection to an access provider's modem.

Protocols related to TCP/IP include the User Datagram Protocol (UDP), which is used instead of TCP for special purposes. Other protocols are used by network host computers for exchanging router information. These include the Internet Control Message Protocol (ICMP), the Interior Gateway Protocol (IGP), the Exterior Gateway Protocol (EGP), and the Border Gateway Protocol (BGP).

Getting started with TCP/IP

To explore how TCP/IP is used in the enterprise, here are some additional resources:

Understanding TCP/IP: This tutorial explains TCP/IP, including history, development, its use as a standard, Requests for Comment and the protocol suite.

Security and the TCP/IP stack: This article explains the security mechanisms, protocols, and applications associated with each layer of the TCP/IP stack.

Test your TCP/IP protocol stack to troubleshoot network connectivity: When you troubleshooting Windows network connectivity, make sure there are no problems with the local TCP/IP protocol stack. Brien M. Posey guides you step-by-step through testing the local host's ability to contact the DNS server and the default gateway server, and how to test host name resolution.

Troubleshooting TCP/IP and network device integration issues: There are many facets of TCP/IP that are optional or may be configured in different ways that can interfere with device integration. This tip will help you take a panoramic view of TCP/IP and help you identify interactive points that can be problematic.

Learn more about Network Administration

Do you have something to add to this definition? Let us know. Send your comments to techterms@whatis.com

More resources from around the web: - The Internet Protocol is formally described in IETF RFC 791. - The Transmission Control Protocol is formally described in IETF RFC 793. - SearchNetworking.com offers a list of white papers about TCP/IP and other network technologies. - The team of experts at searchNetworking.com can answer your questions about TCP/IP.

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