- Port
- 445
- Protocol used by the service
- TCP
- IANA registrations
- TCP/UDP
- Encryption
- Varies by configuration
- IANA description
- Microsoft-DS
- IANA service name
- microsoft-ds
Note: IANA has formally registered SMB on TCP/UDP, but in real-world practice this service operates over TCP only — a formal registration on a transport doesn't guarantee the protocol is actually implemented or used over it. This page's Protocol field reflects genuine usage, not just the registry entry.
What port 445 is used for
Windows file and printer sharing, plus related services like Active Directory. Runs directly over TCP on this port (an older variant ran over NetBIOS on ports 137-139 instead).
Is port 445 encrypted?
SMB has supported encryption (SMB 3.0+) for years, but it is not always enabled by default depending on OS version and configuration — plain, unencrypted SMB is still common on internal networks.
Firewall & exposure considerations
This is the protocol the EternalBlue exploit and the 2017 WannaCry ransomware outbreak used for wormable lateral movement, as documented in CISA's WannaCry advisory (see References below). Should never be exposed to the open internet — restrict it to trusted internal networks only, and keep systems patched.
Common questions about port 445
Is port 445 TCP or UDP?
SMB uses TCP port 445. IANA registers 445 for both TCP and UDP, but SMB itself runs over TCP. The one newer exception is SMB over QUIC, available on Windows Server 2025 (and Windows Server 2022 Datacenter: Azure Edition) with Windows 11 clients, which uses QUIC instead of TCP; Microsoft lists 443 as its default port, so it does not use port 445 by default.
What is the difference between port 445 and port 139?
Port 445 carries SMB directly over TCP. Port 139 is the NetBIOS session service, the older transport SMB was carried over, and it works together with UDP 137 (NetBIOS name resolution) and UDP 138 (NetBIOS datagram service). Windows Server 2025 reflects the shift away from them: the restrictive 'File and Printer Sharing' firewall group it creates for new shares no longer permits inbound NetBIOS ports 137-139.
Should port 445 be reachable from the internet?
No. Microsoft's guidance is that perimeter firewalls should block unsolicited traffic from the internet, and outgoing traffic to it, on SMB's ports, including TCP 445 and UDP 137 and 138, and that where SMB is needed, rules should allow only the specific endpoints involved. CISA's 2017 WannaCry alert advised, where the patch could not be applied, disabling SMBv1 and blocking all versions of SMB at the network boundary: TCP 445 plus UDP 137-138 and TCP 139, on all boundary devices.
Related guide: Common ransomware ports: what to check and close
Related ports
References
- Microsoft [MS-SMB2]: Server Message Block (SMB) Protocol Versions 2 and 3
- CISA: Indicators Associated With WannaCry Ransomware
- IANA registration for port 445
- Microsoft: Preventing SMB traffic from lateral connections and entering or leaving the network
- Microsoft Learn: SMB features in Windows and Windows Server
- Microsoft Learn: Ports that are used by Remote Desktop Services
Look up any other port
Search the full IANA registry — all 6,100+ ports and 12,500+ protocol-specific records — on the Port Lookup tool.
Last reviewed: . Data source: IANA Service Name and Transport Protocol Port Number Registry. Security and usage notes are this site's own editorial content, not official IANA data.