Local NetBIOS Name Query Broadcast Not Forwarded by Router

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This article was previously published under Q150881

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SUMMARY

Routers do not normally forward b-node broadcasts. NetBIOS over TCP/IP uses b-node broadcasts for name resolution and registration on your local LAN segment. If you want your router to forward b-node broadcasts, you must enable UDP ports 137 and 138.

MORE INFORMATION

A b-node broadcast is a UDP datagram. If these broadcast datagrams pass through your router, it will greatly increase traffic on your internetwork and decrease performance. Not every router can forward b-node broadcasts, and usually this function is disabled on routers that can forward the broadcasts.

By enabling UDP ports 137 and 138 on your router, a name query broadcast with the destination NetBIOS name can be forwarded. RFC 1002 defines UDP ports 137 and 138 as follows:

Port Number       Key name
137/UDP           #NAME_SERVICE_UDP_PORT

138/UDP           #DGM_SRVC_UDP_PORT

For additional information, please see the following article in the Microsoft Knowledge Base:

136403 (http://support.microsoft.com/kb/136403/EN-US/ ) Description of UDP Ports

NOTE: Forwarding UDP broadcasts of ports 137 and 138 may cause browsing problems in some environments. "Inter-Networking with Microsoft TCP/IP Using Microsoft Windows NT, Participant's Workbook," P/N 4271B, pages 144-147.