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Check for open TCP ports
TCP/IP communications between your computer and other computers on the Internet pass through numbered logical ports on each computer.
Knowing which ports are open on your computer can help you assess the security of your system or troubleshoot connection problems.
Open Network Utility and click Port Scan.
Enter your IP address, then click the Scan button.
To find your IP address, choose Apple menu > System Preferences, click Network, then select a network service, such as Ethernet or Wi-Fi.
For more information on ports and the software that uses them, see the Apple Support article TCP and UDP ports used by Apple software products.