What are ports?
A port is a name given to an endpoint of a logical connection. Port numbers identify types of ports. For example, on the Internet, port 80 (HTTP) is used to connect computers for the purpose of showing and viewing web pages. So, when you type in your domain name (http://domainname.com) the page is shown by connecting your computer to a webserver via port 80. The fact that you're using http:// before the domain name signals that we want to look at port 80, or whatever port is assigned to HTTP.

If you're unsure of port assignments, then you should refer to the list below, as these are the most typical ports used by default.

PORT #Service Service Description
7EchoEcho Server -- Returns what is sent.
9DiscardA sink - like /dev/null
15NetstatA variant of systat - for remote management
21FTPFile Transfer Protocol -- Copy files from one server to another.
23TELNETAllows remote logins
25SMTPSimple Mail Transfer Protocol
53DNSDomain Name Server -- Converts Domain Names to IP Addresses and back again.
69TFTPA very simple protocol used to transfer files
70gopherPrecursor to HTTP
79fingerProtocol provides an interface to a remote user information program
80HTTPHyper Text Transfer Protocol -- For viewing Web Pages.
87LINKTalk/Chat style protocol
110POP3Post Office Protocol -- For downloading Email to your local computer.
119NNTPNetwork News Transport Protocol
143IMAPMail protocol used by Sendmail, Qmail, etc. -- For viewing Email while keeping it on the remote server.
161SNMPSimple Network Management Protocol -- For Sending Email
443HTTPSEncrypted form of HTTP - Default port for encrypted transmissions
513Loginwho/rlogin - Sends out broadcast giving list of who is logged in
514rsh(tcp) Port for remote shell connection
514SYSLOG(udp) System log facility
515printerCommon port of unix printers
515syslog(udp) Alternate port to 514 for syslog messages
5190ICQChat Program
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