Cổng:18— Message Send Protocol

Port 18 sits in the System category. This page lists the registered purpose and the software commonly associated with it; to see what is actually listening on a specific machine, check the process with ss/netstat.

The registered protocol on this page is TCP. The same number behaves independently on each protocol: TCP may be listening while UDP is idle, so check them separately.

On Linux: sudo ss -tlnp 'sport = :18' for an exact port match. On macOS: sudo lsof -nP -iTCP:18 -sTCP:LISTEN. On Windows PowerShell: Get-NetTCPConnection -LocalPort 18 -State Listen. A listener counts as confirmed only when you can see the bind address and process name; without root/admin you will not see other users' processes.

A port number has no inherent security — it depends on whether the listening service is correctly configured and whether it needs exposure at all. Keep loopback/internal-only services firewalled; where public access is required, pair it with source restrictions, VPN or authentication.