How to check if port is open a step by step guide

When we talk about port checker we need to think both ways, outgoing and incoming after all the server side need to have some kind of a service to listen on the desired port.

check if port is open
check if port is open

This can be called an open port, due to the fact that it open, ready for connections.

Let’s look deeper on the server side.

A service is running waiting for connections on a special port, for example, a web server like IIS or Apache will listen on port 80 for HTTP and port 443 for HTTPS as the default ports.

A service is a software, an exe file under windows operating system. Please note that it can be part of a tool and include as a service under the services on the operating system, but it can also be an individual exec file that runs on the system.

The service term, in this case, is to represent a software that service one or more remote connections.

Once our service is up and running we can use a build in command line tool call netstat to verify that the port this service use is open.

The command you need to know – netstat:

If we run the command netstat -an | find “port number” 
If we see any result, we can assume that the port is open and the service is waiting for connections.

But what if we are behind a firewall, like on a company, or your home router. This means that our server IP address is within the company hidden from the web. When we use it for the web the router gives us another IP address for that.

We need to configure our router (also apply for firewall) for port forwarding, means that if someone from outside our company, like you from home, will try to connect to that service, the IP that you can see is the router IP address, so what can we do?

In the router we add a rule that will forward any connection that tries to connect on the port we define to the server that holds the service, once we do it we can use a client from outside to test it.

An easy to use client tool, that we can use to can check if the port is open is an online port checker or an online open port checker, like this site.

We will set the IP address and the port and if we get a respond – open, we know we succeeded. we just did a port forwarding test – cool!

Summarize the server side:

  1. Service to open the port and wait for connections.
  2. Verify that the required port is open.
  3. Configure for port forwarding in the router and firewall.
  4. Use an external tool, like online open port checker to verify we can reach the service (aka connect).

Please note that in a company there are more steps that need to be done. Including the security team approval, the network team to prepare all the needed configuration and so on.

Let’s look deeper on the client side.

The client can a standalone exec file, a web browser or mobile apps that need to connect the server.
If it runs from a PC we may need to allow it to send data out of our system and to accept incoming data from the server.

This can be done if needed by opening or allowing this tool in your local firewall.
Most of today firewall will automatically allow it to transfer data over the internet if the client is the one that initializes the connection.

For the common question is my port open?
Well if you were able to connect the remote server – yes, it is (assuming the server is configured as needed).
You can use the build in command line telnet tool to check if the remote port is open.

Using the telnet command:
telnet If the result is a blank screen with or without text in it, the port is open.

If you want to use a GUI based tool here are 2:

Putty – PuTTY is an SSH and telnet client. PuTTY is open source software. Its available with source code. It is developed and supported by a group of volunteers. (developed originally by Simon Tatham for the Windows platform.)

And we have our own windows tool that you can use: Windows port check tool


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