Fritz!Box 7390 behind firewall

How to setup a Fritz!Box 7390 for VoIP behind a Netgear UTM, which is connected to a Cable modem.

Fritz!Box Overview

Shows 2 telephone numbers enabled. One landline and other VoIP.

The challenge was how to allow the LAN connected computer to log into the Fritz, but still be protected by the Netgear UTM/Firewall. This solution places the Fritz in parallel to the firewall, but with DHCP turned Off on the Fritz. The firewall has DHCP enabled, and is the gateway for all PCs (and Macs).

Both the Fritz and the Firewall plug into a cable modem/router. The LAN subnet on this site is

The Fritz setup is shown below...


LAN Topology


Internet Settings



  1. Fritz is set to fixed IP, which is outside the DHCP setting of the firewall
  2. NAT is enabled, but without DHCP, as clients should get DHCP from the firewall

VoIP Account Settings

Firewall settings are standard. Start DHCP above the Fritz!Box's fixed IP.


It is also quite workable to put the Firewall behind the Fritz and then PC's would log into the Fritz via the firewall. Downside is that we would have triple NAT-ted our PC connections, with possible performance downgrade.
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