Yes, it is. You don't need Filemaker Server if you have less than 10 simultaneous users.
You don't need a hosting service (but you can use one if you want).
And, dependent on the web-connection speed, it works brilliantly, in my experience. As the file gets bigger you may have to adjust some layouts to avoid sorts on thousands of records, but it is a brilliant facility.
See here for a similar post. http://forums.filemaker.com/posts/50698fdfd4
BUT: that post is to host files and access them using IWP (bear with me...). The same principles about port forwarding work, as explained by David. With IWP it is port 591 that is forwarded, with Filemaker Client access it is port 5003.
On the remote computer, launch the Filemaker client. Choose File -> Open Remote... and choose 'Favourites' (as opposed to 'Local Hosts'). Click 'Add' to create a new connection. Type in the IP address that the router shows to the outside world (not the internal IP address of the computer hosting the file). Type a name for it if you want.
You will then see all of the files that are being hosted on that computer, and can access them as if you were on the same LAN.