How to Check if Port is Open: A Complete Guide

How to Check if a Port is Open: A Complete Guide

Have you ever wondered how your computer communicates with other devices on the internet? How does it know where to send and receive data packets? How does it ensure that the data reaches the right destination and application?

The answer lies in ports. Ports are logical endpoints of communication between two devices on a network. They allow data packets to be sent and received by specific applications or services.

But why do you need to check if a port is open? Well, there are several reasons why you may want to do that. For example, you may want to:

Troubleshoot network connectivity issues
Test the security of your network or device
Configure firewall rules or port forwarding settings
Access remote services or devices
Monitor network activity or performance

In this article, we will show you how to check if a port is open on Windows, Mac OS, and Linux using built-in tools and third-party applications. We will also explain what ports are, how they work, and what are the common types of ports and their functions.

What is a Port and Why Do You Need to Check It?

A port is a number that identifies a specific application or service on a device. It ranges from 0 to 65535. Each port can be associated with either TCP (Transmission Control Protocol) or UDP (User Datagram Protocol). These are protocols that define how data is transmitted over the internet.

TCP ensures reliable and ordered delivery of data, while UDP provides fast and efficient delivery of data without guaranteeing reliability or order.

For example, when you browse the web, your browser uses port 80 or 443 to communicate with web servers. When you send or receive an email, your email client uses port 25 or 587 to communicate with mail servers.

Checking if a port is open means verifying if the port is available for communication. An open port means that the application or service that uses that port is listening for incoming connections. A closed port means that the application or service that uses that port is not listening for incoming connections.

Checking if a port is open can help you diagnose network problems, test network security, configure network settings, access remote resources, or monitor network traffic.

TCP vs UDP Ports: What’s the Difference?

As we mentioned earlier, ports can be associated with either TCP or UDP protocols. But what’s the difference between them?

TCP and UDP are two different ways of sending data over the internet. TCP is more reliable but slower, while UDP is faster but less reliable.

TCP works by establishing a connection between two devices before sending data. It also checks if the data was received correctly and resends it if necessary. TCP ensures that the data arrives in the same order as it was sent.

UDP works by sending data without establishing a connection or checking for errors. It does not guarantee that the data will arrive at all or in the same order as it was sent.

Some common TCP ports and their functions are:

Port 21: FTP (File Transfer Protocol)
Port 22: SSH (Secure Shell)
Port 23: Telnet
Port 25: SMTP (Simple Mail Transfer Protocol)
Port 53: DNS (Domain Name System)
Port 80: HTTP (Hypertext Transfer Protocol)
Port 110: POP3 (Post Office Protocol version 3)
Port 143: IMAP (Internet Message Access Protocol)
Port 443: HTTPS (Hypertext Transfer Protocol Secure)
Port 445: SMB (Server Message Block)

Some common UDP ports and their functions are:

Port 53: DNS (Domain Name System)
Port 67 and 68: DHCP (Dynamic Host Configuration Protocol)
Port 69: TFTP (Trivial File Transfer Protocol)
Port 123: NTP (Network Time Protocol)
Port 161 and 162: SNMP (Simple Network Management Protocol)
Port 500: IKE (Internet Key Exchange)
Port 137, 138, and 139: NetBIOS

You can also check more detailed explanation of essential ports here.

How to Check if a Port is Open on Windows

If you are using Windows, there are several ways to check if a port is open on your device. One of the easiest ways is by using the Netstat command. Netstat is short for network statistics. It shows protocol statistics and the current TCP and IP network connections.

To use Netstat, follow these steps:

Open the Start menu and type “command”.
Right-click on the Command Prompt app and select Run as administrator.
Type netstat -ab and press Enter.
You will see a list of results, depending on what’s currently connecting to the network.
You will see a list of running processes and their associated ports.
The open port numbers will be after the last colon on the local IP address (the one on the left).
Look for the items on the list with a state of “LISTENING”. These are the processes that are communicating via one of the currently opened ports.
If you want to know the program’s name that has a specific port open, then type netstat -aon and press Enter.
This command will show the protocol the app is using, the local and remote IP addresses, and most importantly, the PID of the application using that port (the number on the far right).
Remember to look for the LISTENING status.
To find the application related to that PID, right-click the task manager and select Task Manager.
Select the Details tab.
Look in the PID field for the PID you noted from the command prompt screen.

Another way to check if a port is open on Windows is to use a third-party app. There are many free and easy-to-use apps that can scan your network for open ports. Some of them are:

TCPView: TCPView is a utility included in Microsoft Sysinternals that shows you a list of all running processes and their associated open ports. This app displays ports opening and closing and packet transfers, all in real-time.
Nirsoft CurrPorts: Nirsoft CurrPorts is another utility to see all ports currently being used by applications and services on your system. Just look for the local ports column to see which of your computer’s ports are active. The list also includes ports that are being connected to on the remote end (the server out on the internet).
PortQry Command Line Port Scanner: PortQry Command Line Port Scanner is another command-line utility dedicated to scanning your computer for open ports. You can use it to query a single port, a range of ports, or a list of ports.

How to Check if a Port is Open on Mac OS

If you are using Mac OS, there are several ways to check if a port is open on your device. One of the easiest ways is to use Terminal and Network Utility. Terminal is an application that allows you to access the command line interface of your Mac. Network Utility is an application that provides various network tools, such as Ping, Traceroute, Whois, and Port Scan.

To use Terminal and Network Utility, follow these steps:

Open Finder and go to Applications > Utilities > Terminal.
Type netstat -nr | grep default at the prompt and press Return. The router’s IP address appears next to “default” at the top of the results.
Type nc -vz (your router's IP address) (port). For example, if you wanted to see if port 25 is open on your router, and your router’s IP address is 10.0.0.1, you would type nc -vz 10.0.0.1 25.
Press Return. Here’s how to interpret the results:
If the port is open, you’ll see a message that says the connection succeeded.
If the port is closed, you’ll see a message that says the connection was refused or timed out.
To scan multiple ports at once, you can use a hyphen to specify a range or a comma to separate individual ports. For example, nc -vz 10.0.0.1 20-25 or nc -vz 10.0.0.1 21,23,25.
To exit Terminal, type exit and press Return.

Another way to check if a port is open on Mac OS is to use Network Utility’s Port Scan feature. To use it, follow these steps:

Open Finder and go to Applications > Utilities > Network Utility.
Click on the Port Scan tab.
Enter the IP address or hostname of the device you want to scan in the text box.
Select whether you want to scan only for specific ports or for all ports5. Click Scan.
You will see a list of results showing which ports are open and which are closed.

Another way to check if a port is open on Mac OS is to use a third-party app. There are many free and easy-to-use apps that can scan your network for open ports. Some of them are:

Nmap: Nmap is a powerful and versatile network scanner that can perform various tasks, such as port scanning, host discovery, service detection, vulnerability assessment, and more.
Angry IP Scanner: Angry IP Scanner is a fast and simple network scanner that can scan IP addresses and ports in any range.
Zenmap: Zenmap is a graphical user interface for Nmap that makes it easier to use and visualize its results.

How to Check if a Port is Open on Linux

If you are using Linux, there are several ways to check if a port is open on your device. One of the easiest ways is to use Terminal and Netstat command. Terminal is an application that allows you to access the command line interface of your Linux system. Netstat is a command that displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.

To use Terminal and Netstat command, follow these steps:

Open Terminal by pressing Ctrl+Alt+T or by searching for it in your applications menu.
Type netstat -tulpn | grep LISTEN at the prompt and press Enter.
You will see a list of results showing the processes that are listening on TCP ports.
The open port numbers will be after the last colon on the local IP address (the one on the left).
The process name and PID will be in the last column.
If you want to know more details about the process, you can use the ps command with the PID as an argument. For example, ps -f 1234.
To exit Terminal, type exit and press Enter.

Another way to check if a port is open on Linux is to use a third-party app. There are many free and easy-to-use apps that can scan your network for open ports. Some of them are:

Nmap: Nmap is a powerful and versatile network scanner that can perform various tasks, such as port scanning, host discovery, service detection, vulnerability assessment, and more.
Angry IP Scanner: Angry IP Scanner is a fast and simple network scanner that can scan IP addresses and ports in any range.
Zenmap: Zenmap is a graphical user interface for Nmap that makes it easier to use and visualize its results.

Conclusion

Checking if a port is open can help you diagnose network problems, test network security, configure network settings, access remote resources, or monitor network traffic.

Depending on your operating system, there are different ways to check if a port is open on your device. You can use built-in tools such as Netstat, Terminal, or Network Utility, or you can use third-party apps such as Nmap, TCPView, or Angry IP Scanner.

We hope this article has helped you learn how to check if a port is open on Windows, Mac OS, and Linux.