What is port 139?

After connecting and authenticating, the target's client is disconnected and SMBRelay binds to port 139 on a new IP address. This relay address can then be connected to directly using "net use \\192.1.1.1" and then used by all of the networking functions built into Windows. The program relays all of the SMB traffic, excluding negotiation and authentication. As long as the target host remains connected, the user can disconnect from and reconnect to this virtual IP.