Restrict access to certain computers to my server thru internet

Hi everybody, this is the deal: I have a web server that I need to be accessed thru internet to certain computers. But those remote computers have dinamic IP and in several cases are connected to internet thru NAT routers. I can’t use VPN either. I was thinking for some kind of security certificate installed on the clients. But at the end I need that no one else outside this certain computers connect or bother to my server. Could anybody help me with any solution or at least an idea?