You can't assign a new public IP yourself. This is done through your ISP and subsequently the IANA. Public IP addresses are what the outside world uses to get to your network. From there, your router generally translates the incoming traffic (using port numbers) to your internal network's addresses. This process is known as NAT (Network Address Translation). So, when you "port forward", you are telling the router to forward any incoming traffic on a specific port to one of your internal IP addresses. Assigning each of your computers to an external IP address is really unnecessary, as NAT can do all of that for you.