A technician is locking down a Windows file server with a host-based firewall. The environment still uses NetBIOS sessions for legacy SMB file sharing. After blocking most inbound traffic, users can ping the server but can no longer open its shared folders from remote subnets. Which single TCP port must be opened to restore the NetBIOS session service required for these shares?
NetBIOS over TCP/IP uses three well-known ports. Port 137 (UDP/TCP) provides the NetBIOS name service, port 138 (UDP) handles datagram traffic, and port 139 (TCP) carries the NetBIOS session service that SMB relies on for older Windows file sharing. Because the issue involves establishing a session to access shared folders, opening TCP port 139 resolves the problem. Allowing port 137 or 138 would not re-enable session traffic, and port 445 is used for direct SMB without NetBIOS, which the legacy environment does not yet support.
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Why is TCP port 139 crucial for NetBIOS session service in this scenario?
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What is the difference between SMB over NetBIOS and SMB over TCP/IP?
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