Indeed, doxfer does appear to be down, hopefully Joe can kick that back up again 
Reverse DNS records would be setup through your ISP/hosting provider.
Generally, you would have one reverse DNS record per IP address.
When you say "URL", I assume you mean "domain".
Personally, I register my domains using GoDaddy, but my DNS and such are all hosted from my own servers – GoDaddy isn’t hosting DNS for any of my domains.
I do use GoDaddy in order to register two of my IP addresses as glue records / nameservers, but it’s the servers running Virtualmin that are hosting all of the DNS for my domains.
-Eric