Port 81 is an Internet socket port that the Internet Protocol Suite uses to establish host-to-host communications. These ports keep Internet connections organized and separated, and are set by the Transmission Control Protocol, or TCP. Some ports may also be set by the User Datagram Protocol, or UDP. Your IP address has 65,535 of each type of port, but most of them don't see use. Like Port 81, many Internet ports are used only when another port doesn't work properly.
Port 80 is the default Web server port on most computers. When a user types your host name or IP address in his browser URL field, the browser automatically looks to Port 80 for a connection. However, some ISPs and website owners block this port. If the browser finds that Port 80 has been blocked, it will move on to the next port, looking for a connection.
Port 81 acts as an alternate Web server port when Port 80 is unavailable. You can specifically direct your browser to look for Port 81 instead of 80 by typing ":81" after the domain name in your browser's URL field. You can try this if a site does not seem to be responding on the default port.
Because Port 80 is the default Web port for most sites, viruses, worms and other network-based attacks often target it. Some Internet service providers block Port 80 entirely to prevent this, choosing to use Port 81 or another similar port instead. Other providers block only certain types of traffic from Port 80, allowing outgoing data, but denying incoming data or restricting it to a particular protocol. If you need to communicate with a website when Port 80 has been blocked for security reasons, trying Port 81 may work.
Port 81 sees little use and is noteworthy only for its proximity to Port 80, according to Gibson Research Corp. Other alternate ports for Web traffic include Port 81, Port 8080 and Port 8090. Unless the primary Web port has been blocked, there is little reason to prefer Port 81. Avoid using it unless you encounter problems with a particular website.