Many telnet clients also include a third option, the port on
which the connection should take place. Normally, port 23 is the
default telnet port; the user never has to think about it. But
sometimes it's desirable to telnet to a different port on a system,
where there may be a service available, or to aid in debugging a
problem. Using
telnet somewhere.domain port
will connect the user to the given port on the system
somewhere.domain. Many libraries use this port method to offer their
facilities to the general Internet community; other services are also
available. For instance, one would type
telnet martini.eecs.umich.edu 3000
to connect to the geographic server at the University of Michigan
(Geographic Server). Other such port connections follow the
same usage.