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What is TCP/IP? How does it work?

TCP/IP stands for "transfer control protocol over internet protocol". It is the basic communication language or protocol of the Internet. Actually, it represents a group of protocols called the "TCP/IP suite." This suite includes the HyperText Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet which lets you logon to remote computers, and the Simple Mail Transfer Protocol (SMTP). These protocols are used for transferring data in an organized, standard method over networks of all shapes and sizes. TCP/IP was originally developed by the Department of Defense's Advanced Research Projects Agency, for use on their group of networks called ARPANet. It is the standard for internetwork communications.

When your computer is set up to access the Internet, it is provided with a copy of the TCP/IP "stack". Every other computer that you may send messages to or get information from also has a copy of TCP/IP. It is a two-layered program. The higher layer, Transmission Control Protocol (TCP), controls the transfer of IP data packets. It verifies the transmission of data by checking checksums and signalling starts and stops, and the retransmission of lost data. TCP takes care of keeping track of the individual units of data (called packets) that a message is divided into for efficient routing through the Internet. For example, when an HTML file is sent to you from a Web server, the TCP program layer in that server divides the file into packets, numbers the packets, and then forwards them individually to the IP program layer. Although each packet has the same destination IP address, it may get routed differently through the network. At the other end (the client program in your computer), TCP reassembles the individual packets in order and waits until they have arrived to forward them to you as a single file. TCP is known as a connection-oriented protocol, which means that a connection is established and maintained until such time as the message or messages to be exchanged by the application progams at each end have been exchanged. In the Open Systems Interconnection (OSI) communication model, TCP is in layer 4, the Transport Layer.

The lower layer, the Internet Protocol (IP) is the protocol which determines how information is packeted and directed over the Internet. It 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. Each computer (known as a host) on the Internet has at least one address that uniquely identifies it from all other computers on the Internet. When you send or receive data (for example, an e-mail note or a Web page), the message gets divided into packets. Each of these packets contains both the sender's Internet address and the receiver's address. Any packet is sent first to a gateway computer that understands a small part of the Internet. The gateway computer reads the destination address and forwards the packet to an adjacent gateway that in turn reads the destination address and so forth across the Internet until one gateway recognizes the packet as belonging to a computer within its immediate neighborhood or domain. That gateway then forwards the packet directly to the computer whose address is specified. Because a message is divided into a number of packets, each packet can, if necessary, be sent by a different route across the Internet. Packets can arrive in a different order than the order they were sent in. The Internet Protocol just delivers them. It's up to TCP to put them back in the right order.

An IP address has two parts: the identifier of a particular network on the Internet and an identifier of the particular host (which can be a server or a workstation) within that network. On the Internet itself - that is, between the routers that move packets from one point to another along the route - only the network part of the address is looked at. Each network must know its own address on the Internet and that of any other networks with which it communicates. To be part of the Internet, an organization needs an Internet network number, which it can request from the Network Information Center (NIC).

In addition to the network address or number, information is needed about which specific machine or host in a network is sending or receiving a message. So the IP address needs both the unique network number and a host number (which is unique within the network). (The host number is sometimes called a local or machine address.) Once a packet has reached the right network, that network's routers look at the local address to route to the final 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.)


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