How do I create an irule that rewrites a request to be translated to an ipaddress and port

I'd also add here that the Host header would only reflect the real server's IP if the VIP address is the same as the server's IP, in which case you'd probably have a wildcard 0.0.0.0 VIP with no translation. The user agent will use the host name or IP it uses to access the site as the Host header. So if the user accesses the VIP with "www.example.com", then the Host header would be "www.example.com". You can certainly change the Host header to reflect the IP of the server that the traffic is going to, but it may be worthwhile to examine if that's even required by the server.