TCP/IP Overview

TCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication protocol of the Internet. It can also be used as a communications protocol in a private network (either an intranet, where parties within a single organisation can communicate, or an extranet, where parties may communicate with each other over an external but private network).

The two parts of the protocol, TCP and IP, were developed by the US Department of Defense (DOD) to solve the problem wherein a number of different networks, designed by different vendors, needed to connect with each other in what was, in essence, a network of networks. This network of networks became the Internet.

TCP/IP benefits

TCP/IP provides basic but high-demand services, such as file transfer and electronic mail (e-mail), across a very large number of client and server systems.

Several computers in a small company department can use TCP/IP (along with other protocols) on a single Local Area Network (LAN). The IP component provides routing from the department to the company network, then to regional networks, and finally to the global Internet.

TCP/IP is designed to be highly robust and automatically recover from any network node or phone line failure. In addition it employs flow control mechanisms, to allow for inadequacies of the receiving computer, and support for the detection of errors and lost data, with its ability to trigger retransmission until the correct data is correctly and completely received.

TCP/IP and the OSI 7-layer reference model

TCP/IP is a two-layer program, where TCP is the higher layer and IP the lower layer.

TCP sits in the Transport Layer of the OSI 7-layer model and provides all the packet and error-handling services of that layer.

IP sits in the Network Layer of the OSI 7-layer model and is responsible for the routing of data packets through the network. Each gateway computer on the network checks the packet address to see where to forward the message. Just as in X.25, packets from the same message may be routed differently but will be reassembled at the destination.

IP addresses

Every machine on the internet, or within an intranet or extranet system, has a unique identifying number, known as an IP address. Internet IP addresses are globally unique and assigned by the Network Information Centre, whereas intranet and extranet IP numbers only need to be unique within their own network and can be assigned by an administrator.

The format of an IP address is a “dotted decimal number” such as:

216.27.61.137

The fact that internet IP addresses are globally unique allows IP networks anywhere in the world to communicate with each other.

The four numbers separated by decimal points are used to create classes of IP addresses that can be assigned to particular entities, such as businesses or governments, based on size and need.

Each IP address is divided into two sections: Net and Host. The first of the four numbers is always used to identify the network (Net) to which a computer belongs. The last of the four numbers is always used to identify the actual computer (Host) on the network. The second and third numbers may be used to identify the Net or the Host, depending on which class the IP address belongs to.