Page Forwarding......bleg

I would use google, but (as you might guess) putting anything like “page forwarding” into google doesn’t help, and it’s late, so i’ll ask here.

I need to forward a domain to a subdomain, but keep the domain there, ie

a.com forwards to a.b.com, but a.b.com/c would appear as a.com/c.

Meh. I think it’s JS.