How to find open and blocked TCP / UDP ports


Chances are you are reading this article because an application you are trying to run indicates that the port is blocked, or you have read the documentation that leaving certain ports open on the network can cause security problems.

In this article, Quantrimang.com explains TCP / UDP port is and how to check your computer for open or close the gate.

What is TCP / UDP port?

The two common types of ports on modern networks are called TCP and UDP. These two types of ports use different network protocols.

Both types of ports are built on the basic Internet Protocol (IP) that makes the Internet and home network work well. However, they are suitable for different applications.

The big difference is that when you send information over UDP, the sender doesn't have to establish a connection with the recipient before starting the conversation. This is almost like sending a letter. You don't know if someone else will receive your message, and there is no guarantee that you will receive any response.

On the other hand, TCP is more like making phone calls. The receiver must receive connection data and have a flow of information back and forth until someone deliberately hangs up.

UDP messages are typically broadcast over the network to anyone listening on the specified UDP port. This makes it the perfect choice for messages related to network streaming, Voice over Internet Protocol (VoIP) streaming, video games and broadcasting.

These applications benefit from UDP's low latency and constant flow of information. TCP is much more common than UDP and completely guarantees that all data is received without errors.

Which ports are usually open by default?

There are many ports. The port number can range from 0 to 65535 ! But that doesn't mean any application can choose an arbitrary port, as there are standards and scopes set.

Ports 0 - 1023 are associated with some of the most important and basic network services. This makes sense, as lower numbered ports are pre-assigned. For example, the SMTP protocol for email is used exclusively by port 25.

Ports 1024 - 49151 are known as registered ports and assigned to critical common services, like OpenVPN on ports 1194 or Microsoft SQL on ports 1433 and 1434.

The rest of the port numbers are known as dynamic or private ports. These ports are not reserved and anyone can use them on the network to support a particular service. The only problem that arises is when two or more services on the same network are using the same port.

While it is not possible to list all the important ports, the following are common and very useful ones to know:

  • 20 - FTP (File Transfer Protocol)
  • 22 - Secure Shell (SSH)
  • 25 - Simple Mail Transfer Protocol (SMTP)
  • 53 - Domain Name System (DNS)
  • 80 - Hypertext Transfer Protocol (HTTP)
  • 110 - Post Office Protocol (POP3)
  • 143 - Internet Message Access Protocol (IMAP)
  • 443 - HTTP Secure (HTTPS)

Since there are thousands of common port numbers, the easiest approach is to memorize ranges. This will let you know if a certain port is reserved. Thanks to Google, you can also look up which service uses a specific port in no time.

Find open ports in Windows

The port is open in WindowsThe port is open in Windows

You should now have all the basics of TCP and UDP ports. It's time to move on to finding out which ports are open and used on your computer.

Refer to the article: Check the port is open, the connection is being made in the system for details.

Scans for blocked ports

The above section focuses on finding out which port is being used by which application, but does not tell us which port is being blocked by Windows Firewall .

  • Again, open Command Prompt with admin rights .
  • With Command Prompt open, type:
 
netsh firewall show state

The port is being blocked by Windows FirewallThe port is being blocked by Windows Firewall

Currently, using the show state command is the quickest and easiest way to get portal information.

Windows Firewall doesn't block a port doesn't mean the router or the ISP does. So it's worth checking if any external blocking is going on.

  • Open Command Prompt as administrator.
  • With Command Prompt open, type:
 
netstat -ano | findstr -i SYN_SENT

Check out external blockingCheck out external blocking

If you don't see anything listed, no ports are blocked. If some ports are listed, it means that they are blocked. If a port that wasn't blocked by Windows shows up here, you may want to check your router or send email to your ISP, if you can't switch to a different port.

Useful application to find out the port status

While the Command Prompt is a useful tool, there are more sophisticated third-party applications that can help you get a quick look at your port configuration. The two options suggested here are just common examples.

SolarWinds Free Port Scanner

SolarWinds Free Port ScannerSolarWinds Free Port Scanner

SolarWinds requires you to submit your name and details for the download, but you have a choice whether to include your actual information in a form. Maybe you've tried some free tools before reaching out to SolarWinds, but it's the only one that works well in Windows 10 and has an easy-to-use interface.

This is also the only option that doesn't trigger fake virus warnings. One of the big problems with port scanning software is that security companies tend to see them as malware. So most users will ignore any virus warnings that come with such tools. SolarWinds really works as advertised and is very easy to use.

CanYouSeeMe

CanYouSeeMeCanYouSeeMe

CanYouSeeMe is a website, not an app. It has a great first port to see if external data is accessible through your local port. It automatically detects the IP address, and all you have to do is specify which port to check.

The tool will then tell you if the port is blocked, and next you have to find out if the blocking takes place at the computer, router or service provider level.

For most users, ports are not something you need to worry about. They are managed by the operating system, applications, and network hardware.

However, when something goes wrong, you should have tools at hand to help find open ports to detect suspicious activity or find out exactly where your precious information is in trouble.

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