telnet to a specific port
Telnet, by default, uses port 23. Ports are specific channels for network services and clients to use so that you can have more than one connection at once. Eg telnet works on port 23, ftp works on port 21, web(http) works on port 80 etc. Think of them as numbered doors into the same house. There is nothing to stop you using telnet (or some other services) on another port eg telnet into a web server by using:
telnet www.somewhere.com 80 or www.somewhere.com:80
While this may not be very useful unless you know how to run a specific protocol by hand (some people use this to check their mail on pop servers from very bare terminals) it is a quick way of checking that a certain service is running on a remote machine. If you can't telnet in on the specific port number then there's a good bet it isn't running.
A list of services and the ports they run on are contained in /etc/services. (σε linux)
Some services, even generic ones, may be run on different port numbers on certain machines for reasons of security or convenience and knowing about port numbers is helpful in this instance.
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