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Common Networking Protocols

One of the foundational points of knowledge you need to understand for the Network+ certification are the common networking protocols. A protocol is a uniform set of rules for how systems can connect and communicate with one another. Different protocols use different ports, or a common location that TCP transmits data to. This way, when you request a web page, for example, your computers knows exactly where to connect to on the webserver and the webserver knows where to respond to.

This table provides common networking protocols and what default port they respond to (this is by no means an exhaustive list, just a representation of what you need to know for the exam):

You should be aware of this list at the very least for the exam. You may receive a specific question asking which port a particular protocol communicates on.

This is the default list of protocols and the ports they communicate on. It is not, however, the definitive list. Protocols are not restricted to a certain port, so you could, for example, have your SMTP server communicate on port 125, 225, or 10025. The two systems communicating need to know which port to communicate on and then the communication can occur. If you simply changed your SMTP server to something other than the default (port 25), other SMTP servers will not know to communicate with your server on that port unless they are instructed to.

So, you might set your SMTP server to communicate on an alternative port for a specific reason. For example, if you install a SPAM filtering application, it may communicate on the default port for SMTP, port 25, and then relay the SMTP of good mail to the SMTP server which may be communicating on an alternate port, such as port 225.

Only one application or service can communicate on a single port at one time – so you cannot have two SMTP servers running on the same physical network connection (rather more specifically, the same IP address) occupying the same port – only one service can communicate on a single port at a time.