How Can I Detect Whether A Port Is Open Or Closed?

You can easily test your system for vulnerabilities to Internet threats using on-line tests provided by several security sites. We recommend www.grc.com and www.pcflank.com for this.

Note that in some cases, for example when address translation (NAT) is used in your network configuration or you are using a proxy, those sites will detect your computer's IP address incorrectly. Your IP address can be obtained by running ipconfig.exe (or winipcfg.exe from the Windows command line). If the IP address determined by a test is not your true IP address, cancel the test as further results from that test will be incorrect. For stronger assurance that a port is open or closed visit other scanning sites.

Important: You should also keep in mind that netstat.exe and the Open Ports category in the Lavasoft Personal Firewall's left pane cannot be used for detecting whether a port is open or not. Netstat does not correctly interpret open ports ('listening' in terms of netstat can simply mean 'waiting for an inbound connection'). Also note that information displayed in the Open Ports category in Lavasoft Personal Firewall's left pane lists those ports that are being monitored by the firewall at the moment, but not all of them can be actually open on the network.

Important: Besides, some ports can be blocked by provider (for example, 139, 137, 135, 80) which makes the results of online scanning of those ports impartial.