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Why do I need to specify port numbers in my Internet applications?

Answer: Most Internet connections are based on the TCP/IP protocol. This protocol provides a list of port numbers that are used for different services. When an application communicates over a certain port, it expects a specific type of connection with another system or application. Specifying port numbers ensure two systems communicate correctly. Below are a list of common port numbers and their associated services.

  • 20 - FTP (file transfers)
  • 25 - SMTP (mail servers)
  • 80 - HTTP (websites)
  • 443 - HTTPS (secure websites)
  • 993 - IMAP over SSL (secure email transmission)

While it is possible for two systems to communicate over any given port, it can cause interference if other data is being transmitted on the same port. Therefore, the ports listed above, as well as several others, are standardized for different services. This allows developers to specify the correct default ports for their Internet applications. It also makes it easy for network administrators to monitor or block specific ports for security purposes.

Online games and web server applications often use custom ports, which are not standardized. In order to avoid conflicts with other ports, most custom ports are numbered 1000 or higher. For example, Blizzard's World of Warcraft uses ports 3724, 6112, 6113, 6114, and 4000. If you need to choose a custom port for your Internet application, it may be smart to first look up the port using a search engine to make sure it is not being used by another popular program.

Published: November 15, 2011 — by Per Christensson

Answer from the PC Help Center