By using DNS forwarders you can improve the efficiency of name resolution for the computers in your network that query for DNS names outside your network (such as names on the Internet).
From a network and performance efficiency perspective, having a single forwarder is generally more efficient than using multiple forwarders. Whenever a DNS server receives a query and resolves it using forwarders (as with Root Hints), it caches the results it sent back to the client.
Since query results are concentrated in one forwarder’s cache, any subsequent queries to the same names will be resolved locally from that DNS’s cache.
Make sure to clean up the cache by executing (ipconfig /flushdns) on client.
Then try without any forwarder configured it, you should have issues to hit external sites.
You also get the benefit of having the ISP cache most of the frequently-used DNS queries for your country or geographical region in their DNS servers’ cache, further improving DNS query performance.
In all these cases you will need the correct names and IP addresses of your ISP.
In most cases the DCs (or at least some of them) will also act as DNS servers.
When you make a change to these DCs, you must remember to also change the forwarders, firewall rules and any other manual configuration settings you made.
You want to have more than one DNS server for obvious redundancy purposes.
When you have two or more DNS servers, you can configure one of them, some of them, or all of them to use forwarders.
The best ways to get the correct names and IP addresses of your ISP would be either to search for the list on your favorite search engine or simply contact your ISP’s technical support line.