TCP Port 445 — Server Message Block (SMB)
July 25, 2026
TCP port 445 is the registered port for direct-hosted Server Message Block (SMB) traffic, the protocol Windows systems use for file sharing, printer sharing, and general inter-process communication over a network. SMB originated at IBM in 1983, authored by Barry Feigenbaum, as a way to expose local DOS file access over a network; Microsoft and 3Com carried it into LAN Manager for OS/2 around 1990, and Microsoft's 1996 fork, renamed CIFS, added symbolic-link support and larger file sizes. Early SMB ran encapsulated in NetBIOS over NetBIOS Frames or IPX/SPX; port 445 exists because SMB was later given a direct transport over TCP/IP, bypassing NetBIOS entirely. SMB 2.0, introduced with Windows Vista in 2006, cut the protocol's command set from over a hundred operations to 19 and added request pipelining to reduce round trips. SMB 3.0, released with Windows 8 and Server 2012, added end-to-end encryption, multichannel operation, and RDMA support; SMB 3.1.1 (2016) strengthened this with AES-128-GCM encryption and SHA-512 integrity checks. Authentication is handled via NTLM or Kerberos, with optional message signing to guard against man-in-the-middle tampering. SMB remains the transport underlying Windows file and printer sharing and Microsoft's Distributed File System.
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