Only one application at a time can listen on a given port
number. If you pick a port number like 80 or 8080 (two ports
commonly used by web servers) and you have a web server running on your machine, your HL7 server will throw some type of
exception on start up because it will not be allowed to listen to the same
port.
Using netstat -a is a really helpful command to run from
a command prompt on your computer to see what applications are actively
listening on what TCP/IP ports. Consult your operating system documentation
for more information.