Port 138 supports NetBIOS Datagram Service and netbios-dgm (TCP reference coverage) across distinct transports. Review each protocol before writing firewall or monitoring policy.
Connectionless NetBIOS datagrams used by legacy Windows networking.
Legacy browsing, announcements and file-sharing support.
Bidirectional
Keep within trusted legacy segments and retire where direct-hosted SMB and DNS meet application needs.
netbios-dgm (TCP reference coverage) is documented as a network service convention for port 138 over TCP.
Use this reference entry when identifying or validating systems documented to use 138/TCP.
Varies
Confirm the listening process and current vendor documentation before permitting traffic; a listed convention does not prove the active service or its security posture.