What is the best way to place DNS servers in your intranet to serve your users'needs? Many DNS servers exist to hold copies of an organization's zone files, as you've seen if you've ever set up a DNS server. But a lot of DNS servers hold no zones, living only to resolve names, whether on the Internet ("What is the IP address of https://www.microsoft.com/en-us/?") or on your intranet ("Where is the nearest DC for acme.com?"). Such a DNS server is called a caching-only server. After you set up a zoneless DNS server, you can see a reference to its caching-only nature in the event log (event ID 708).
A caching-only server's strength lies, as its name implies, in the fact that DNS servers remember the results of previous resolutions. For example, if someone in your office points his or her Web browser to http://www.cnn.com, the Web browser asks its preferred local DNS server to find the IP address of http://www.cnn.com from CNN.com's DNS server. The preferred local DNS server goes out on the Internet to get that information, and that process takes time.
But the second person to ask the local DNS server for http://www.cnn.com's IP address gets a nearly immediate response because the server resolves the name out of its cache rather than turning again to the Internet for the answer. However, the local DNS server will eventually return to CNN.com's DNS server to determine whether http://www.cnn.com's IP address has changed.
The reason for the return trip is that when the CNN.com DNS server responded to the initial query, the response included not only http://www.cnn.com's IP address, but also the amount of time that the local DNS server should cache that IP address. That amount of time is called the Time to Live (TTL). All responses to DNS resolution requests contain a TTL. After the TTL expires, a new query causes the local DNS server to return to the Internet to resolve the name.