Some routers will allow you to reach the external IP from inside your network, some may not. Mine currently does which is great (I can access www.example.com which resolves to my external IP address and it is forwarded as though I were outside my network).
What I do is register public domains on "example.com" and I have a sub-domain for internal IPs (home.example.com). My local DNS server responds for home.example.com and forwards for the rest.
This makes it clear to me which address I will be getting. And since all my public web traffic goes through a reverse-proxy it lets me also have a name for the server itself. e.g. "music.example.com" may go through the reverse proxy for my music server, but "music.home.example.com" resolves to my music server directly. So I can use the latter for ssh and other things.