Port 137 netbios-ns

NetBIOS name service (UDP)

firewalls: Firewall administrators will frequently see large numbers of incoming packets to port 137. This is due to the behavior of Windows servers that use NetBIOS (as well as DNS) to resolve IP addresses to names using the "gethostbyaddr()" function. As users behind the firewalls surf Windows-based web sites, those servers will frequently respond with NetBIOS lookups.