Below are the methods that I think of, but ISA Server couldn't do the routing
1) (router)***.208.240.1<--->***.208.240.2(ISA Server)***.208.240.3<--->Internal network (public IP addresses in the network ***.208.240.0, subnet mask: 255.255.255.224)
2) (router)192.168.1.1<--->192.168.1.2(ISA Server)***.208.240.1<--->Internal network (public IP addresses)

Anyone who knows the way?
Thanks