Ports

A port can be thought of as a channel of communications to a machine. Similar to telephones, it is like a company PABX that has several lines. Packets of information coming into a machine are addressed not only to that machine, but to that machine on a specified port. You can think of a port as a radio channel if you like, but the fundamental difference between a radio receiver and a computer, is that the computer can listen to any / all of 65000 possible channels at once!

However, the thing to note here is that typically the computer is not listening on very many ports at all. The computer will not respond to data or connection requests that come in on a port that it is not listening to.

One other thing to note is that there are a number of important predefined ports which are universally used for various services. The major ones of these are:

Service	Port#	Description
FTP	21	File Transfer Protocol - for transferring files
Telnet	23	for logging into an account on a Remote Host
SMTP	25	For Sending mail
Gopher	70	Text menu based browser
HTTP	80	WWW protocol - Netscape, Mosaic
POP 3	110	Downloading Mail
NNTP	119	Internet Newsgroups
IRC	6667	Internet Relay Chat
Compuserve	4144	Compuserve WinCIM communications
AOL	5190	America Online
MSN	569	Microsoft Network

More ‘used’ ports are listed in the Appendix.

IMPORTANT NOTE

Two applications on one computer generally cannot both listen on the same port at the same time. This is relevant in that if you try to set WinGate to listen on a port that some other application is already listening on (i.e. you are already running an FTP server), it will not be able to start listening.

WinGate note

If you find you get the error icon when you start WinGate ‘Cannot start socket listening’, then you can be pretty sure this is the reason. What you then need to do is check in WinGate to see which local port has come up as disabled, and change that service to listen on a different port. Port numbers above 1024 are normally safe to use. Remember to update the client applications that were relying on that service being on that port to the new port number.