User Datagram Protocol (UDP) provides a connectionless, unreliable transport service. It is often used for one-to-many communications, using broadcast or multicast IP datagrams. Because delivery of UDP datagrams is not guaranteed, programs using UDP must supply their own mechanisms for reliability if needed. Microsoft networking uses UDP for logon, browsing, and name resolution.
UDP is used for (1) NetBIOS name resolution by using unicast to a NetBIOS name server (such as WINS) or subnet broadcasts, and (2) for DNS host name and IP address resolution. NetBIOS name resolution is accomplished over UDP port 137. DNS queries use UDP port 53. Because UDP itself does not guarantee delivery of datagrams, both of these services use their own retransmission schemes if they receive no answer to queries. Broadcast UDP datagrams are not usually forwarded over IP routers, and so NetBIOS name resolution in a routed environment requires a name server of some type, or the use of static database files.
Many NetBIOS programs use mailslot messaging. A 2nd class mailslot is a simple mechanism for sending a message from one NetBIOS name to another over UDP. Mailslot messages may be broadcast on a subnet, or may be directed to the remote computer. In order to direct a mailslot message to another computer, there must be some method of NetBIOS name resolution available. The WINS server running under Windows NT Server provides this service.