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.
Port forwarding or port mapping allows remote computers to connect to a specific computer or service on a private network. This allows you to run a web server, game server or a service of your choosing from behind a router.
In a typical network the router has the public IP address and computers/servers obtain a private IP address from the router that is not addressable from outside the network. When you forward a specific port on your router, you are telling your router where to direct traffic for that port. This utility can verify the success of that process.
Please refer to your routers manual or manufacturer for assistance in setting up port forwarding.
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.