In the system shown in the figure, all devices at the apple.com site are configured to use the local DNS server. Similarly, all devices on the example.com network are configured to query the local DNS server of that site. In this example, imagine that Apple has registered its domain name with register.com, and the example.com domain has been registered with yahoodomains.com.

When a device on the example.com network needs to perform a DNS lookup, it queries its onsite DNS server.

If that server can answer the query in some way from its cache, or because it is authoritative for the domain in question it will pass the answer back to the client and the lookups are complete. If, however, the local DNS server does not have the answer, it must perform a recursive query to fetch the answer needed.

A recursive query sends the query up the DNS hierarchy and allows other servers to perform the query on its behalf. The response to the query is ultimately passed back to the originating DNS server, which then passes it on to the client. Imagine that a device on the example.com network needs to look up partners.apple.com.

The device on example.com will not find the IP address of partners.apple.com, neither on its local DNS server nor among the domains listed by its registrar, yahoodomains.com.

To find the correct IP address, the local DNS server queries one of the root name servers. The root name server, in effect, says, “Ah you’re looking for information about a server in the .com domain? I know just the server for you to talk to.” From there, the root server refers example.com’s local DNS server to the generic top-level domain server for the .com zone. This server, in turn, passes back a reference specifically to the DNS server responsible for the apple.com domain.

Now, example.com’s local DNS server can query the server that is authoritative for the apple.com domain, which can actually answer the question. It retrieves the answer to the query, with a forward or reverse lookup, from the hosted DNS service and passes it to the client that originally made the query.

The local DNS server will also now cache this result, allowing it to directly answer this question for any other clients in the future, until the expiration of that record.