This is a free utility for remotely verifying if a port is open or closed. It is useful to users who wish to verify port forwarding and check to see if a server is running or a firewall or ISP is blocking certain ports.
Most residential ISP's block ports to combat viruses and spam. The most commonly blocked ports are port 80 and port 25.
Port 80 is the default port for http traffic. With blocked port 80 you will need to run your web server on a non-standard port.
Port 25 is the default port for sending and receiving mail. ISPs block this port to reduce the amount of spam generated by worms on infected machines within their network.