If you want share files... you must show your IP, because the cannection must be directed to you.
I've a domain on my IP, so I can select "Custom" and set my domain name. But that domain is a valid internet domain and point to my IP real address.
If you type in custom "fuck_address.com" or "1.1.1.1", and that address doesn't exist (or doesn't point to your real IP), everyone cannot access to your HFS.
hmm.. sorry fot the language... I hope that I can help you a little-bit.
