137
NetBIOS name resolution on Windows networks
NetBIOS Name Service
What is port 137?
UDP port 137 is used by Windows for NetBIOS name resolution — mapping computer names (e.g., MYPC) to IP addresses. In modern networks it has been replaced by DNS. When exposed to the internet it can leak hostnames and domain information, and is exploited in NBNS poisoning attacks using tools like Responder. Always block this port at the firewall perimeter.
Quick checks
$
nc -zv example.com 137
$
nmap -p 137 example.com