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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. Weve seen how TCP/IP allows machines to find each other by the use of addresses. 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 are assigned to specific tasks, like sending e-mail, and the packet specifies the port number (function) desired as well as the destination 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 application programs 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 installed to communicate on a specific port number. For most common network utilities, these port numbers are standardized across TCP/IP, and will be the same on all systems (unless the admin changes them, which is not recommended unless youre an advanced administrator). For example, when you receive your email, you are calling the service POP3 (Post Office Protocol Version 3) via Port number 110. When you get your email, your client computer running e-mail client software creates a session, and a socket connection attached 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).
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