General network considerations, DNS, DHCP, Gateways


IMPORTANT: There should only ever be one DHCP server on a network. Using a Draytek/Netgear or similar gateway router is usually OK, but if the device loses power (eg. during a power cut), then it has no storage, and will forget all of the leases. This then introduces the risk of duplicate IP addresses and a broken network. Therefore, if there is a server available, it is far better to use that for DHCP.

The PABX can offer a basic DHCP service, and that is what we recommend.

As documented elsewhere the DHCP server should be configured to supply Option 66 for phone auto-provisioning. The PABX DHCP server has this enabled by default.


In small environments without a DNS server, the ADSL gateway will generally serve DNS directly from the Internet, and as such, it cannot have any knowledge of internal devices, including the PABX. Working DNS is required by almost all VoIP handsets.

The PABX can be used to serve its own DNS information, and act as a forwarder. To do this, configure the PABX to use itself ( as a primary DNS, and the ADSL gateway as its secondary DNS. Using DHCP, point all other devices at the PABX for their DNS services. The PABX will serve its own internal name directly, and pass all other requests on to the gateway.

DO NOT be tempted to use the ADSL gateway’s DNS as well as the PABX DNS, as they will respond with conflicting results for any non-Internet queries.

Multiple ADSL gateways

Having a separate ADSL service for VoIP is a good idea, but requires an understanding of IP routing to implement well. There are 2 common scenarios:

  • The VoIP connection is owned/managed by the client, and is a “normal” business ADSL connection. In this case, set the default gateway of the PABX to point at this VoIP router, leave all other devices pointing at the data connection.

  • The VoIP connection is managed by an ITSP and is heavily QoS’ed, and can only be used for VoIP. In this case set ALL default gateways to point at the Data router, and then on the data router, add a static route so that all packets addressed to the ITSP’s IP address(es) are forwarded to the VoIP router. (In more recent versions of the PABX it is additionally possible to configure up to 3 static routes on the PABX itself. If more routes are needed, use a router as that is a device designed for routing tasks, which the PABX is not)

The second option may appear to put double traffic on the LAN, but in fact IP networking will optimise this out.