Wednesday, September 26, 2007

Test if a port is open

One way is to use old telnet.exe like this to find out if a port is open.

Syntax: telnet [hostname or IP address] [portnumber]

Example: telnet myserver 27000

You can also try the IP of the computer, server or host. You must be able to Ping that address.

If the window hangs while saying "Connecting ..." and then goes away, the port is not accessible. If the window instantly goes away, the port is probably not accessible. If the window display text, like "220 ESMTP spoken here" or just shows an empty window the port is open.

Or if run in a command window:
C:\>telnet myserver 27000
Connecting To myserver...Could not open connection to the host, on port 27000: Connect failed

But if you have Windows Vista or Windows 7 you probably don't have telnet because it is not installed as default.

Click Start, Control Panel, Programs, and then Turn Windows Features on or off. In the list, scroll down and select Telnet Client. Click OK to start the installation. Or you can just run: start /w pkgmgr /iu:"TelnetClient"

Now you should be able to check if a port is open or closed or not accessible. If it is not accessible start checking if the firewall is the issue.

Here are some usable links:


  1. Restored comment
    by Matt Mason
    The telnet approach works pretty well for a single port. If you know what you're doing you can even use it to send POP/SMTP/ETC commands.

    But you ought to try "Angry IP Scan" for when you need a broader scan (multiple ports or multiple hosts or both).



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