When connecting the Edge appliance to an existing firewall, you have three options for configuration. Each option works well, it just depends on the
1) Static routing where the Edge routes any existing IP address information directly to the existing firewall (no address changes) NOTE: This requires the ability to access and change the WAN1 gateway router.
2) Proxy or drop-in mode where the Edge appliance sits between the WAN1 gateway and the existing firewall and essentailly bridges the traffic between them (no IP address changes). NOTE: No other devices can be connected between the firewall and the WAN1 gateway router besides the Edge appliance.
3) NAT mode, where the Edge appliance performs NAT on the WAN addresses and One-to-One NAT rules are applied to pass all traffic to the existing firewall. NOTE: Requires that the IP address of the firewall be changed to a private address.