We have covered IP addresses, but what about the TCP part? TCP stands for Transmission Control Protocol, and is the method by which application data is broken down, transmitted, and reassembled across a network.
Remote systems need to connect to each other regardless of underlying hardware or operating system. This is the whole reason for the TCP/IP suite. We have seen how TCP/IP allows machines to find each other by the use of addresses, but since each machine is generally set up to accomplish more than just one task, its not enough to send packets to a specific machine and rely on it to figure out what to do with them. A hint as to the purpose of the packet has to be provided in the packet, to let the target system know what the user wants to do.
The way this hint is provided is that specific port numbers, ranged between 1 and 65535, are assigned to specific tasks such as sending e-mail, and the packet specifies the port number (function) desired as well as the destination IP address of the remote system. When a packet is sent, IP delivers the data from the client to the correct server (as determined by the destination IP address) and the port number is used to make the connection to the particular service in need.
This delivery of data is accomplished by means of applications on each side of the network connection one on the client side, and one on the server side. These programs themselves are generally referred to as clients (for the client-machine side) and servers (as you might expect, for the server-machine side). Typically, each particular service has its own client and server program components.
When the software for a service is installed on a server, it is configured to communicate on a specific port number. For most common network utilities, these port numbers are standardized across TCP/IP implementations and operating systems and will be the same on all systems (unless the administrator changes them, which is usually not recommended unless done for security reasons). For example, when you receive your email, you are calling the service POP3 (Post Office Protocol Version 3) via port number 110. When you retrieve your email, your client computer running e-mail client software creates a session and a socket connection to the servers port 110.
A socket is a software object (not a hardware socket) that is specific to your computer system, which knows how to communicate with a corresponding socket on a server computer that is also running TCP/IP.
Communications can occur regardless of the specific computer or operating systems in use, as long as both the client and server computers follow the same standard for location of ports (such as 110 for email).
Table 35 highlights some of the most important well known services (protocols) and the ports at which to find them. There are many more TCP/IP application protocols available, to serve different specialized purposes.
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