Draytek Vigor2500V Manual page 44

How you operate DNS on the Vigor router?
4.3
The Internet traffic travels between two points addresses by their numeric IP
address. To make the Internet friendly, we are more accustomed to using
names rather than numbers (e.g. www.draytek.com) but those names often
need to be converted back into their real numeric address for the data to be
sent; this is known as "name resolution".
conversion for us so all users on the LAN need to know about a DNS server
in order to resolve addresses.
Your PC will be told of the DNS server to use if your PC is getting its own IP
address automatically from the router (using DHCP). The router will pre-set
DNS servers initially, but the ISP will allocate their own DNS servers which
are then used once the router has connected to the ISP. As a result, you
can over-ride all DNS server settings and force your own by entering them
into the DNS fields on the LAN setup menu
Primary IP Address: Sets the IP address of the primary DNS server.
Secondary IP Address: Sets the IP address of the secondary DNS server.
Your ISP allocates its own DNS servers when the router logs into the ISP.
These DNS servers will then over-ride the manual settings. If you still want
to force the manual settings to apply, you can use telnet command " srv
dhcp frcdnsmanl on" (use "off" to disable it. Requires firmware version
2.5.6 or later)
LAN TCP/IP and DHCP
4-5
A DNS server proceed this