Online Port Scanner allows you to check a host or IP for an open or listening port.

Ports are hardware locations for passing data in and out of a computing device. Personal computers have various types of ports, including internal ports for connecting disk drives, monitors, and keyboards, as well as external ports, for connecting modems, printers, mouse devices, and other peripheral devices. In TCP/IP and UDP networks, port is the name given to an endpoint of a logical connection. Port numbers identify types of ports. For example, both TCP and UDP use port 80 for transporting HTTP data from a web server. Some viruses attack specific ports as part of their design such as SASSER virus that uses TCP port 445 and others.