Networking Fundamentals

Logical ports

When we discussed switches and routers earlier in this book, we mentioned ports. In that context, we were discussing physical ports. A logical port is one that is defined in software. The purpose of a logical port is to allow the receiving device to identify which application or service the data is destined for.

I always find that an analogy is useful here. Imagine you are calling your bank. You ring their main number (the equivalent of the IP address). When the receptionist answers, you ask to be put through to the mortgage department on extension 253 (the port number).

Both TCP and UDP headers include a source port number and a destination port number. When the data is received by the recipient device, the destination port ...