basic firewall: how to unblock services for a second IP address

hi,
i have a VPN set up with RRAS on a single NIC server, it is also a web
server. VPN clients are allocated addresses from a static pool. i
have a basic firewall enabled on the LAN connection, allowing http
traffic on port 80 and https traffic, and a few other services. before
now, the server only had one static IP address and the services and
ports were configured based on this IP address.
i have now added a second IP address to the same LAN interface, and i
find that web traffic is being blocked by the basic firewall. under
Nat/Basic Firewall, i opened the properties of the LAN interface, and
tried to add in a new service called 'Web server, xx.xx.xx.171' with
the new ip address, TCP, on port 80 incoming and outgoing, but i got a
message telling me to "choose a unique special port". i presume this
is because there is already a 'service' set up for http on port 80 on
the existing IP address. how can i unblock services for the new ip
address?

thanks
tim

hi,
just re-posting this in the hope that someone out there can help..
thanks
tim