Port checker is a network service used to check your external IP address and detect open ports on your connection. This tool is useful for finding out if your port forwarding is setup correctly or if your server applications are being blocked by a firewall.
This tool may also be used as a port scanner to scan your network for ports that are commonly forwarded. Some ports, such as port 25, are often blocked at the ISP level in an attempt to prevent malicious activity. To identify available services running on a server, this service uses raw IP packets to find out what ports are open on a server or what OS (Operating System) is running or to check if a server has firewall enabled etc.
The service can also detect uptime of a host, if the host is running one of the known OS which the scanner can analyze to guess uptime. To scan a host just enter the host name or the IP address in the box above and give a range of ports to scan, The recommended scan type is “connect()”, however other scan types can also be useful depending on the network of the target host, a “SYN Stealth” scan type can also prove to be useful when there’s a firewall blocking the ports. Check open ports is possible if your computer’s current external IP address.
More about this you can ask your ISP. Worth considering that if your computer is connected to the Internet, instead of directly through the router, the test results are exactly to the router. Check the port status for the computer inside a subnet is possible only with port forwarding.
Please check this great service at: http://www.test-net.org/services/network-mask-calculator/ or visit FREE SERVICES menu