Draytek Vigor2500V Manual page 45

LAN TCP/IP and DHCP
If both the Primary IP and Secondary IP Address fields are left empty, the
router will assign its own IP address to local users as a DNS proxy server
and maintain a DNS cache. If the IP address of a domain name is already
in the DNS cache, the router will resolve the domain name immediately.
Otherwise, the router forwards the DNS query packet to the external DNS
server by establishing a WAN (e.g. DSL/Cable) connection.
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