Encyclopedia of Computer Science and Technology

TCP/IP 469The header fields and data for an IP (Internet Protocol) packet. Thepackets can travel over different routes to the destination addressand then be reassembled in the correct order.is enabled by having one computer (or a hardware devicecalled a router) connected to the Internet, serving as thelink between the local network and the rest of the world. Afacility called Network Address Translation (NAT) assigns aprivate IP address to each computer on the network. Whena computer wants to make an Internet connection, its outgoingpacket is assigned a public IP address from a pool.When packets replying to that public address are received,they are converted back to the private address and thusrouted to the appropriate user.NAT has the benefit of providing some security againstintrusion, since from the outside only the single public IPaddress is visible, not the private addresses of the variousNetwork Address Translation (NAT) can protect computers on alocal network by giving outgoing packets an arbitrary public sourceaddress in place of a computer’s actual (private) address. Incomingpackets addressed to that public address are then routed to the correctprivate address using a table.machines on the network. However, using NAT (and a similarscheme called PAT that allows difference hosts to use thesame IP address by being assigned different port numbers)causes some slowdown because of the translation process.Another facility, Dynamic Host Configuration Protocol(DHCP), is used to assign an arbitrary available public IPaddress to each host when it connects to the network. Thissystem is now used by most DSL and cable systems, and itreduces the danger of running out of IP numbers (each networkis assigned a range of numbers, and is thus limited tothat many IP addresses).A more lasting and flexible solution to address depletionis the new Internet Protocol version 6 (IPv6). The newaddressing scheme allows for about 3 × 10 38 addresses,more than enough to accommodate every star in the knownuniverse if it were a networked computer! The scheme isbeing rolled out gradually but should be well established bythe end of the 2000 decade.Domain Name SystemInternet users typically don’t have to worry about IP numbers,except perhaps when configuring their software.Instead they use alphabetic addresses, such as http://www.factsonfile.com. The Domain Name System (DNS) sets up acorrespondence between the names (which include domainssuch as .com for commercial or .edu for educational institutions)and the IP numbers (see dns).Transmission Control ProtocolThe Transmission Control Protocol (the TCP part of TCP/IP) controls the flow of packets that have been structured asdescribed above. To use TCP, the sending computer opensa special file called a socket, which is identified by thecomputer’s IP number plus a port number. Standard portnumbers are used for the various protocols such as www(Web) and ftp (File Transfer Protocol). The receiving computerconnects using a corresponding socket. TCP includesbasic flow control and error-checking features similar tothose used for most data transmissions. For some applications(such as connecting to the domain name server) errorcontrol is not needed, so a simpler protocol called the UserDatagram Protocol is used.The Big PictureHow does TCP/IP fit into the use of the Internet? When anapplication such as an e-mail program, Web browser, or ftpclient makes a connection, IP packets using TCP flow controlcarry the requests from the client to the server and theserver’s response back to the client. Each application has itsown protocol to specify these requests (such as for a Webpage). For e-mail the protocol is SMTP (Simple Mail TransferProtocol); Web servers and browsers use HTTP (HypertextTransfer Protocol); and for file transfers it is FTP (FileTransfer Protocol). (See also e-mail, html, hypertext andhypermedia, and file transfer protocols.)Further ReadingHagen, Silvia. IPv6 Essentials. 2nd ed. Sebastapol, Calif.: O’Reilly,2006.