An Open Port Checker is a tool or methods that can be use to check if a port or list of ports are open, In a short, an open port, in computer, is a service that waits for connection aka listen on a special number that represent the port, the door to use in order to get in.
So for example if I want to open a web page I will type the site URL or IP Address number and the web browser will automatically add it the port number 80, which is represent to default port for web server like IIS and Apache.
The following answer, regarding checking if a port is open, is true for all the following common questions that people ask, like:
How to tell if a port is open?
How to determine if a port is open?
How to know if a port is open?
How to know if port is open in windows?
The easiest way to do this, without the need of installing an external tool, is to open the command line and type netstat, this build in tool can give you some nice to know information about the current open ports in your system, you can read this netstat article that give you live example how to use it.
Using telnet/putty as an open port checker tool:
- Telnet: Telnet is a small build in tools in the windows operation system, it is a command line tool, this mean the interface to the tool is from the command line, the easies method to use it is to open the command line, re: bring on the Run windows, type in cmd and press enter, on the command line (the black window) type telnet [hostname] [port] to check the remote port. In later windows operation system the telnet tool is not install by default, search the web how to install it, according to your operation system.
- Putty: Putty is a free tool that you can download from the internet, from this link putty, once you run it you can select the protocol you want to use, like telnet, ssh or other, select telnet, the default port that using by telnet is 23, change it to the port you want to check, after that just press the open button, if it just close, the port is close, if it open black screen the port is open…
An online port scanner tool or a port checker online tool is a tool that scan a port or list of ports using a method to check if the port is open or not, the easy way to verify if a port is open or not is using a “connect()” function with the needed parameter (host name and port) to check the requested port.
Advanced open port checker:
A more advanced method will include a part of a full connect request to check the port status, in a full connect, under TCP, there are 3 steps, known as the TCP 3 handshake, an SYN, SYN-ACK and ACK, so if it only send the SYN buffer and get a SYN-ACK answer i know the port is open, also if i send the SYN-ACK buffer and did not get a time out/error message I also know that the port is open and so on.
To check if port is open command line also may call check if port is open cmd you can use a tool like telnet or putty to check if a port is open on a target server or host, if you need to check your local open port you can list them by using the build in netstat command with the needed parameters to get a list of local open ports.