external network interface

i have instaled MS Proxy 2.0 on win2k server. my server’s ip address is 192.168.1.20 & its gateway is 192.168.1.7 (which is Cisco pix firewall).

now when i click on “security” in webproxy > Properties i get this error

“packet filtering cannot be enabled on this server because it does not have an external network interface”

1)how do i add this external network interface ??

2) am using pix firewall as gateway for my client machines , what should be the gateway for my proxy server ?