All computers connected to the Internet, and most computers connected to local networks run the Internet Protocol to communicate with each other. The most common protocols on IP implement the concept of "port": a number that, combined with a computer's network address, completely identifies a networking application running on that computer. You can determine which (if any) application is listening on a given port at a given address, from the command line provided by a Terminal on Ubuntu Linux. This is commonly referred to as "pinging" the port.
Log in to any computer, as long as it is connected over the network to the computer where the port to be pinged is. Launch a Terminal session by clicking on "Applications," then selecting "Accessories," then "Terminal."
Type the following command into the Terminal:
nmap -p 80 hostname.domainname.com
Replace "80" with the number of the port you want to ping, and "hostname.domainname.com" by the fully qualified name of the host where the port is. Press "Enter."
Search the output of "nmap" for the result of pinging the port. If the port is listed as "closed," there was no application listening at that port. Otherwise, "nmap" will list it as "open" and will make a (usually good) conjecture about the specific application listening at the port; that will be displayed under the "SERVICE" column.
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