Port 139 supports NetBIOS Session Service and netbios-ssn (UDP reference coverage) across distinct transports. Review each protocol before writing firewall or monitoring policy.
Session-oriented NetBIOS transport commonly used by legacy SMB.
Older Windows file and printer sharing implementations.
Client to server
Prefer direct-hosted SMB on 445 where supported and do not expose this port to untrusted networks.
netbios-ssn (UDP reference coverage) is documented as a network service convention for port 139 over UDP.
Use this reference entry when identifying or validating systems documented to use 139/UDP.
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.