UDP is used for NetBIOS name resolution by unicast to a NetBIOS name server or subnet broadcasts, and for DNS host name to 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, so NetBIOS name resolution in a routed environment requires a name server of some type, or the use of static database files.