Port checker is a free online tool to test ports in your system. This tool lets you scan open ports which may prove security holes to hackers. This tool can also test whether port forwarding is working or not.
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.
Port Forwarding is a kind of special configuration on the router, which allows to redirect external requests (from the Internet) to computers or other devices on the local network. In fact it is a way to specify which local computer to send data and connection requests that arrive at a specific port on the router. 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).
For Windows: Start → "cmd" → Run as Administrator → "netstat -bn" For 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).