I used the same domain for chat that I do for email. I then use split-dns.
For example.
the domain is WHATEVER.COM
On my public DNS, I create an A record for xmpp.whatever.com that points to my public ip
I then create my SRV records _xmpp-server and _xmpp-client with a target of xmpp.whatever.com
Next on the internal DNS servers, Create a zone xmpp.whatever.com that points to the internal IP for the openfire server.
Next configure connection manager domain to use whatever.com. Configure openfire to use the domain whatever.com
*note - if this is an existing install, all your JID will change to match the the domain. Be sure to add an admin JID using the new domain.
Let me know if you run into any problems, and I'll do my best to help