not in ohio
reply to Mark
Re: Netbios Session Service - Port 139 It's sometimes important to note that file/printer sharing is just one of a potentially open-ended set of applications using Netbios Session. Netbios Session dispatches connections to particular applications based on a 16-byte name sent in a session setup request; you can see the names in use on a particular machine with the nbtstat -n command.
By convention, Netbios apps use the last byte as a sort of protocol type (just like TCP or UDP apps use a port number as a sort of protocol type). The server side of file/printer sharing uses the machine name with a last byte of 0x20 (ascii space); the clent side of file/printer sharing uses the machine name with a last byte of 0x00 (ascii nul).
From a security point of view, however, if you take the simple approach of screening tcp/139 from the outside world, you don't need to know this extra detail.