TCP stands for Transmission Control Protocol and IP stands for Internet Protocol. TCP/IP is a layered set of protocols.

TCP/IP is the base communication protocol of the Internet. IP part of TCP/IP uses numeric IP addresses to join network segments and TCP part of TCP/IP provides reliable delivery of messages between networked computers.

To understand about TCP/IP then just concentrate on this example. A typical situation is sending mail. Then first, there is a protocol for mail. This defines a set of commands which one machine sends to another, e.g., commands to specify who the sender of the message is, and who it is being sent to, and then the text of the message. However this protocol assumes that there is a way to communicate reliably between the two computers. Mail, like other application protocols, simply defines a set of commands and messages to be sent. It is designed to be used together with TCP and IP. TCP is responsible for making sure that the commands get through to the other end. It keeps track of what is sent, and retransmits anything that did not get through. If any message is too large for one datagram, e.g., the text of the mail, TCP will split it up into several datagrams, and make sure that they all arrive correctly. Since these functions are needed for many applications, they are put together into a separate protocol, rather than being part of the specifications for sending mail.

You can think of TCP as forming a library of routines that applications can use when they need reliable network communications with another computer. Similarly, TCP calls on the services of IP. Although the services that TCP supplies are needed by many applications, there are still some kinds of applications that do not need them.

However there are some services that every application needs. Therefore these services are put together into IP. As with TCP, you can think of IP as a library of routines that TCP calls on, but which is also available to applications that do not use TCP. This strategy of building several levels of protocol is called layering.

Usually, TCP/IP applications uses the following four layers:

  • an application protocol such as mail
  • a protocol such as TCP that provides services need by many applications
  • IP that provides the basic service of getting datagrams to their destination
  • the protocols needed to manage a specific physical medium such as Ethernet or a point to point line

TCP/IP is based on the catenet model. This model assumes that there are a large number independent networks connected together by gateways. The user should be able to access computers or other resources on any of these networks.

Datagrams will often pass through a dozen different networks before getting to their final destination. The routing needed to accomplish this should be completely invisible to the user. As far as the user concerned, all he need to know in order to access another system in an Internet address.

The Internet address looks like 128.64.193. It is actually a 32-bit number. However it is normally written as 4 decimal numbers, each representing 8 bits of the address.

The term octet is used by Internet documentation for such 8-bit chunks. The term byte is not used, because TCP/IP is supported by some computers that have byte size other than 8 bits.

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