TCP/IP is a two-layered 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 "connectionless" 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 connectionless frees network paths so that everyone can use them continuously. (Note that the TCP layer itself is not connectionless 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 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 a 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).

   Different terminal emulation is required for specific types of terminal - for example, the IBM 3270 display terminal or the AS/400's 5250 display terminal . The program performing the terminal emulation must understand the data stream from the mainframe at several communication levels, including data link control and session control. Special hardware may be required, depending on how the PC using the terminal emulation program is being connected to the mainframe - for example, through remote dial-up, leased line, or ISDN.

   Some terminal servers can be shared by up to 128 terminals. The terminals can be PCs, terminals that emulate 3270s, printers, or other devices with the RS-232/423 interface. In some terminal servers, the terminals can use TCP/IP for Telnet connection to a host, LAT to a DEC host, or TN3270 for Telnet connection to an IBM host with 3270 applications. With some terminal servers, a given terminal user can have multiple host connections to different kinds of host operating systems (UNIX, IBM, DEC).

The term communication server is also sometimes used instead of terminal server.

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