Implementing CIFS: The Common Internet File System

Port 139 is for NBT, and port 445 is for raw TCP good rules of thumb. Recall, though, that the NBT Session Service provides a mechanism for redirection. In addition, some security protocols use high-numbered ports to tunnel SMB connections through firewalls. That means that the use of non-standard ports should be supported on the client side.

The SMB URL allows the specification of a destination port number, like so:

smb://server:1928/

Once again, that fits into standard URI syntax. If you spend any time using a web browser, the port field should be familiar.

What this all means, however, is that the port number does not always indicate which transport should be used. Rather the opposite ; if the port number is not specified, the default port depends upon the transport. Knowing which transport to choose is, once again, something that requires some figuring out.