How to Determine Open and Blocked TCP/UDP Ports

Looking for a way to determine which ports are open and which ports are blocked on a computer? Ports are basically logical connections between two computers or network devices for sharing data. Port numbers range from 1 to 65536 and some port numbers are assigned to specific tasks, such as port 80 for transferring HTTP data.

Port 21 is for FTP, port 25 is for SMTP, port 110 is for POP3, port 23 is for Telnet, etc, etc. There are many ports that, if left open, can be considered a high security risk. Many viruses spread to different computers because of open ports, such as the Sasser virus, which used port 445 (Windows File Sharing) to infect thousands of machines. Read my previous article on how to prevent remote connections to your PC.  

There are several ways to check for open ports, my favorite being using port scanner tools. They are free and you don’t have to install any software to scan for open ports. Also, you can scan any computer as long as you know the IP address. If you are a real geek or hardcore admin, you can use Netstat to check for open ports.  

A really good port scanner is T1 Shopper, which lets you scan a single port, a range of ports, or the most common vulnerable ports like FTP, NetBIOS, etc.

online port scanner

By default, it puts in the IP address of your current Internet connection, so you can test your home or office computer to see if which ports are listening. The web service will run through each port test and print out a line showing whether there was a response or not.

check for open ports If you are looking for a way to test your web server or website to see which ports could be open, you can try out a cool tool called OPScanner 2.0. Just type in the URL of the website you want to test and type in the range of port numbers to scan and click Scan Target.

open ports As you can see, this website only has port 22 open, which is Secure FTP, so it’s very secure. Of course, I only scanned from 1 to 50 in this case. Using both of these tools, you can quickly see if there are unnecessary services running on your computer or server that could allow a hacker to gain unauthorized access. Enjoy!

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Comments [2]

  1. [...] How to determine open and blocked TCP/UDP ports Posted on Saturday, October 11th, 2008 in Networking – Views: (127) Looking for a way to determine which ports are open and which ports are blocked on a computer? Ports are basically logical connections between two computers or network devices for sharing data. Port numbers range from 1 to 65536 and some port numbers are assigned to specific tasks, such as port 80 for transferring HTTP data.Port 21 is for FTP, port 25 is for SMTP, port 110 is for POP3, port 23 is for Telnet, etc, etc. [...]

  2. al says:

    There is free tool for that: porttest.blogspot.com/.

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