Internet Video Series, Part 2
Episode 14, Segment 02 of 08
First, let's clarify the differences between - "CLIENTS" - and "SERVERS".
When we say - "SERVER", you can think of a great big, listening ear, because - SERVER - processes spend their idle time constantly listening for incoming requests. They are always ready to answer questions addressed to them if they are written in a language and format that they understand. Like other processes in your computers, they are always assigned a - "Process ID number" - by your operating system. By longstanding convention, communicating processes are assigned numbers, known as - "PORTS" - through which client processes can reach them with their inquiries or requests. It is commonplace to say that a server process is - "listening on" - some TCP or UDP port. Even when running simultaneously on different computers in different Internet locations, the best-known, longest-established server processes are almost always assigned the same Port numbers inside each host, which eventually become - "well known", contributing to the informal, evolving standardization of Internet communication. Thus it is easy for clients to find the well-established servers.
When we say "CLIENT", you can think of a great big - "Question Mark", because - CLIENT - processes are activated on the network when somebody needs information or needs a question answered. Unlike - SERVER - processes (which are generally left running all the time), - CLIENT - processes are generally shut down when they are not needed. Operating systems generally assign unpredictable, random-looking - "Process ID" - and - "Port" - numbers" - to client processes, and they change from instance to instance. When a client process sends an inquiry to a server process on a well-known port number, the inquiry message always includes the port number on which it expects to receive an answer. Server processes use this port number information in combination with the client's IP address to ensure that their response information is delivered to the right place.
Episode 14, Segment 03 of 08