The Internet makes use of many ports for connectivity, this is a necessary part of the networks that compose the global network and enables all kinds of network protocols to work together. Here are some of the most common ports in use.
- Echo: 7 – echo is used to test the communication between hosts. Any data that is sent to port number 7 is echoed back to the host.
- Daytime: 13 – Responds to any connection with the time of day on the server.
- FTP: 21 – used for transferring files.
- TELNET: 23 – Used for connecting to a remote machine over a network. Not encrypted and not secure.
- SMTP: 25 The port used for Simple Mail Transfer Protocol. Used for transferring E-Mail over a network.
- WHOIS: 43 A directory service for looking up users on a remote server. This is commonly used to get registration information for a domain.
- Finger: 79 Displays information about a user logged into a server.
- HTTP: 80 This is the port that everyone uses. Web servers serve out requests on port 80 when giving out a HTML page.
- POP3: 110 This is the Post Office Protocol 3, this is used to retrieve stored E-Mail from a mail server.
- NNTP: 119 Network News Transfer Protocol. This is used by the venerable usenet network to transfer news and files.
- HTTPS: 443 Secure HTTP Protocol. This is HTTP running on top of the Secure Sockets Layer (SSL) for secured web browsing.
For a more comprehensive list of all network ports; there is a list of them on Windows machines in the C:\Windows\System32\drivers\etc\services file. For those of us on other operating systems there is a list on Wikipedia: http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers.