How the internet works:
12.3.2. Zone File Resource Records
The primary component of a zone file is its resource records.
There are many types of zone file resource records. The following are used most frequently:
A — Address record, which specifies an IP address to assign to a name, as in this example:
If the value is omitted, then an A record points to a default IP address for the top of the namespace. This system is the target for all non-FQDN requests.
Consider the following A record examples for the example.com zone file:
IN A 10.0.1.3
server1 IN A 10.0.1.5
Requests for example.com are pointed to 10.0.1.3, while requests for server1.example.com are pointed to 10.0.1.5.