The first page is where your router connects to so the instance of dnsmasq running on it, as a resolver, can lookup names doing DNS service for the computers and devices connected through it on the local network. Usually, it gets populated by DHCP from the settings given to you by your ISP or the gateway provided to you by your ISP.
The second page is where you want the computers and devices to goto to find the DNS server.
Leaving it blank defaults to the DHCP server telling clients to use the router for DNS lookups.
On your computer or phone you can see that the DNS provider inside your network is pointed to the IP address of the router.
If you were running a DNS-Blackhole (Pixel-server, Pi-Hole or other variants on your network) you'd put their IP addresses there for name resolution.
Open-DNS offers custimizable DNS services , as do several other sites that offer ad-block, family , children, malicious site protection.
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