What is port 445 used for?
Port 445 is SMB, the Windows file and printer sharing protocol. Mapped drives, shared folders and many Windows management functions between computers in a domain all travel over it.
Inside a network 445 is essential: domain members use it to read Group Policy and logon scripts from domain controllers. From the internet it should be unreachable. Microsoft's guidance is to block TCP 445 at the perimeter in both directions, because exposed SMB has been the route for worms; the flaws in the old SMBv1 version that Microsoft patched in 2017 with bulletin MS17-010 are the best-known case. Current hardening is to remove SMBv1, require SMB signing and use SMB 3 encryption for sensitive shares.
Is port 445 TCP or UDP?
TCP. SMB runs directly over TCP 445. The newer SMB over QUIC option uses UDP 443 instead, so that file access can cross the internet without exposing 445.
Is SMB on port 445 secure?
It depends on the version. SMBv1 is obsolete and should be removed. SMB 3 supports signing and end-to-end encryption. Whatever the version, port 445 should be blocked at the internet edge and allowed only between the machines that need it.
Port 445 vs port 139: what is the difference?
Both carry SMB file sharing. Port 445 is SMB directly over TCP, used by current Windows versions. Port 139 is SMB inside a NetBIOS session, kept only for very old systems. Ports 137-139 explained
Which CompTIA exams list port 445?
CompTIA lists port 445 by number in both the Network+ and the A+ Core 1 objectives.
- Network+ N10-009: objective 1.4 (Networking Concepts), listed as “Server Message Block (SMB), 445”.
- A+ Core 1 220-1201: objective 2.1 (Networking), listed as “445, Server Message Block (SMB)/Common Internet File System (CIFS)”.
The quoted text is how the objectives document lists the port. The exam puts the port in a scenario, such as a firewall rule or a failed connection; it does not only ask for the number.
Check yourself: a practice question
Written by CertSprint for this page. It is not taken from any exam.
A perimeter firewall review finds TCP 445 open from the internet to a file server. What is the correct action?
- Block 445 at the perimeter and provide remote file access through a VPN
- Move the share to port 139
- Leave it open but hide the server name
- Change SMB to UDP 69
Show the answer
A. Block 445 at the perimeter and provide remote file access through a VPN. SMB should never be exposed to the internet. Remote users reach shares through a VPN or a service designed for the internet, such as SMB over QUIC.
Sources: IANA Service Name and Transport Protocol Port Number Registry: port 445; Microsoft Learn: Secure SMB traffic in Windows Server; Microsoft Learn: SMB security enhancements; Microsoft Learn: SMB over QUIC; Microsoft Learn: Security Bulletin MS17-010. The port number and how each exam lists it come from CompTIA's exam objectives, linked from the study plans above. The explanation and the question are CertSprint's own, checked against these sources on 12 November 2026.