Port checker is a free online tool to perform Port Forward Test on your machine .To test for an open port you need to enter domain name like "xyz.com" or IP Address in the first box and desired port number e.g 80 in second box. On press of Check button system will take some time and show you the results at the bottom search section.
Let's say you have a home game or a web server connected to the Internet via a router. All computers connected to the same router are on the same network, so will be able to connect to the server. However, outside of the Internet, connect to your server without port forwarding will not work. If your computer is connected to the Internet directly (without a router / router), then perform port forwarding is required. All your open ports must be accessible from the Internet (of course, if you have a dedicated IP).
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 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.
In case you have set up DMZ host and you want to test whether your firewall is effective of not same can also be verified by port checker tool. Use your publich ip and test the ports which you have configured to block from your firewall.
Ports that virtual address on your computer or network device which enables it communicate with other computer or devices. If a program or service plans to work with the network, it opens the port with a unique number, through which it can work with remote clients / servers. In human language it would sound something like this: "I, the server program, open port 1234. If the network cable will come with data port number 1234 - that to me."
Ports are identified by numbers from 0 to 65535, inclusive. Any other port can not be opened, respectively, and check too. This limitation TCP / IP protocol. It is worth noting that the client program should always know the port to which it needs to connect to a remote server or another network device. For this reason, the most popular protocols reserved ports in the range from 0 to 1023. For example, carrying out surfing on the Internet, your browser connects to port 80 on a remote server that hosts the site. In response, the browser receives a set of data and code that downloads and displays a web page.
If you using a Windows or Linux machine, you can easily find open ports or ports which are in use using below commands :
Windows : Start → "cmd" → Run as Administrator → "netstat -bn"
Linux : a In the terminal, run the command: "ss -tln"
First of all it is necessary to remove the cause - a running program or service that opened the port; it is necessary to close / stop. If the open port reason is not clear - Antivirus scan your computer, delete the extra port forwarding rules on the router and set the advanced firewall (Firewall).