Open Port Check Tool – Test Port Forwarding on Your Router
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. It is important to note that some ports, such as port 25, are often blocked at the ISP level in an attempt to prevent malicious activity.
In a typical network the router has the public IP address and 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 tool can verify the success of that process.
If you’re looking for a full list of port numbers check out this wikipedia page.