A port is like a doorway. Specific services have an associated port number along with the protocol used for communication.
For example email uses 2 ports:
The first service is used to transport email to and from other servers. This is
accomplished with the Simple Mail Transfer Protocol (SMTP).
The SMTP service application usually listens on TCP port 25 for incoming requests.
The second service is the Post Office Protocol (POP) which is used by e-mail client
applications on user’s personal computers to fetch email messages from the server.
The POP service listens on TCP port number 110.
Sadly there are many cyber criminals who use a technique known as Port Scanning to
probe home routers like yours for open ports.
In addition there are also computer worms roaming the Internet looking for open ports
– seriously I am not making this up!
So you want to make sure your Router Ports are hidden from all this scanning.
To do so you can use the FREE service from Gibson Research Corporation (GRC) called: