How to Test Whether a Port is Open

Requirements

This document presumes you're using Windows.

Introduction

Sometimes you may get a "connection refused" message for mail, the web, or another service. It can be helpful to see whether the error is coming from the application, the computer, or the server. Testing whether a port is open eliminates the application from the equation.

What To Do

  1. Find out what server and port you need to test: web is usually 80, outgoing mail is 25, and POP is 110, as examples.
  2. Go to the Start Menu -> "Run..."
  3. Type "telnet server-name PORTNUMBER". For example: 
    telnet pop.wfu.edu 110
  4. If the window hangs while saying "Connecting ..." and then goes away, the port is not accessible. If the window instantly goes away, the port may not be accessible. If the window display some kind of text, like "220 ESMTP ..." then the port is open.
  5. If there are problems, try again using the IP address of the machine.
John Borwick