TCP/IP
TCP/IP is a layered protocol similar to the ones used in all the other major networking architectures, including IBM's SNA, Windows' NetBIOS, Apple's AppleTalk, Novell's NetWare and Digital's DECnet. Layering means that after an application initiates the communications, the message (data) to be transmitted is passed through a number of stages, or layers, until it actually moves out onto the wire. The data are packaged with a different header at each layer. At the receiving end, the corresponding programs at each protocol layer unpackage the data, moving it "back up the stack" to the receiving application.

TCP/IP is composed of two parts: TCP (Transmission Control Protocol) and IP (Internet Protocol). TCP is a connection-oriented protocol that passes its data to IP, which is a connectionless one. TCP sets up a connection at both ends and guarantees reliable delivery of the full message sent. TCP tests for errors and requests retransmission if necessary, because IP does not.

Some of the most widely known application protocols in the TCP/IP suite are FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), Telnet, DNS (Domain Name System) and WINS (Windows Internet Name System). FTP programs are widely used to copy files across the network. All TCP/IP-based mail programs use SMTP to send e-mail. Telnet is a terminal emulator that provides access to a remote host. DNS and WINS allow hosts to be given understandable names, and the DNS and WINS servers turn those names into the IP address required by TCP/IP networks.

All nodes in a TCP/IP network (clients, servers, routers, etc.) are assigned an "IP address," which is written as four numbers between dots, such as 193.4.64.01. The first part of the address is the network address, and the second part is the host (station) address, also known as the netid and hostid. The network address allows TCP/IP packets to be routed to a different network. Every client and server station must have a unique IP address. Client workstations have either a permanent address or one that is dynamically assigned to them each dial-up session.