configuring isa server 2004 using a proxy ?

[shahidrizwan]

currently we are using a proxy (192.168.45.90) for internet on a network scheme 192.168.30.0-254. how should i configure the new ISA2004 installation on separate PC with in the existing network, with new IP range like 192.168.31.0-254 using that proxy 192.168.45.90.