modify entire permalink including website URL

What code (.php procedure) assigns the post i.d. (that includes the permalink) to new posts?

Thanks!

p.s. Here’s why I’m asking:

My web hosting service allows me to have multiple domains on one account, but only the primary domain supports MySQL. So, in order for me to set up WordPress on my secondary domains, I used the domain’s fixed I.P. address.

This works fine (in fact, it seems to be the only way WP will work in this situation) except for the fact that the generated links include the IP (which, of course, makes sense) instead of the domain name.

I’d like to have WordPress assign a post id that uses the domain name and not the IP. I already have a PHP routine in place that will convert the domain-name-based link back to an IP-based link.

My goal is that users only see the pretty name in the links.

Thanks again.