This article describes how to find and monitor open TCP ports in Windows operating systems. There are many different scenarios in which knowledge of open TCP ports is extremely important.

What are TCP ports?
Every Windows server and client provides TCP ports through which a TCP connection can be established. Even if a server still responds to a ping, it is possible that a connection is no longer possible on those TCP ports. Checking the TCP ports periodically is a good way to determine if a server is working properly. In most cases, TCP ports are made available by Windows “Services”.

How can I find open TCP ports on a server?
Here we refer to the device that makes TCP ports available as a "server". This can also be a Windows client, a router or switch or any other device that communicates via TCP/IP in the network.

► Option 1: netstat command on the Windows client
With the command netstat -a from the command line, you can see which connections are currently established. This allows you to see on which TCP ports the client is communicating with the servers. If, for example

TCP 10.5.1.12 CRM-SERVER:9001 ESTABLISHED

is displayed, you know that your own client is talking to the CRM-SERVER on TCP port 9001. If “LISTENING” is displayed for Status, your client provides a TCP port on the relevant port number to which other clients can connect.

The client provides a TCP port on port 3389 (RDP).


► Option 2: netstat command on the Windows server
You can see the connections on the server in the same way - if you have administrative access to the server. Simply enter netstat -a  in a command prompt.


► Option 3: PortCheck from the client
The free mini command line tool PortCheck.exe can be used to check a server for open ports from the client. Basically, you can use it to query individual ports, but you can also use the command

PortCheck.exe MyServer 1-50000 or
PortCheck.exe MyServer knownports

to check entire ranges. TCP ports that are displayed here in green are responding.

Checking the known ports in Windows 11 - only Port 22 (SSH) answers

Important TCP ports:
21 (FTP - File Transfer Protocol), 22 (SSH), 23 (Telnet), 25 (SMTP), 80 (HTTP), 110 (POP3), 135 (DCE / WMI), 139 (NetBIOS), 143 (IMAP4), 443 (HTTPS), 445 (SMB), 1433 (MS SQL), 1494 (ICA - Citrix), 1521 (Oracle), 3389 (RDP)

>>> Change to the TCP Port check PowerShell page
>>> Change to the german Offene TCP-Ports anzeigen page