Computer systems use TCP and UDP for most of their communication, and all versions of Windows open many ports that provide useful functionality such as file sharing and remote procedure call (RPC).

Check what ports are open in Windows 11/10 Open Command Prompt, type the following, and hit Enter to view the Ports used along with the Process Identifiers (PIDs): netstat -ano This will display ...

With no firewall rule to open TCP port 25 dnscheck.net reports a timeout when connecting on port 25. If I add a firewall rule to open TCP port 25 checkdns.net reports that the connection was refused.

Industrial IoT systems using open communication protocols may be vulnerable to attacks through TCP Port 502. Barracuda's NextGen Firewalls can help protect against that.