138
NetBIOS datagram broadcasts on Windows networks
NetBIOS Datagram Service
What is port 138?
UDP port 138 is the NetBIOS Datagram Service, used by Windows for LAN broadcasts such as the browser service, logon notifications, and similar announcements. It is no longer necessary on modern networks that rely on DNS and Active Directory. This port should always be blocked at the internet perimeter, as it exposes internal Windows network structure.
Quick checks
$
nc -zv example.com 138
$
nmap -p 138 example.com