If you use Server for DNS with a Domain Controller, set up a forwarder to the DNS addresses of your WAN (internet connection). If you set up your modem connected to any computer directly with the network card set to dynamic connection settings, you should be able to see the dns by oppening the connection and getting the Details. Then just plug modem into network card A on server or into router with static LAN address, set up DHCP on server and plug card B on server into network switch; make DHCP server call router or CARD A the router\gateway (using the Ip Address you get from Details), and link the dns name with forwarders as your domain name for DHCP. Then point the DNS server to Card B interface. This will have server DNS forward the lookups from the internet, and link your computers to your server with the suffix of your Domain name, while your Card A or router is set as the Gateway.
If you have Static IP settings, you should have the IP Address of the gateway (Static ip address on internet, not default Gateway of your WAN), and you should also have your dns lookups. Use them similarly, but set your card a or router to have those settings first.
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