What relationship does server1 IN A 192.168.1.10 define?
server1 IN A 192.168.1.10
It declares server1 as the authoritative nameserver.
server1
It maps the hostname server1 to the IPv4 address 192.168.1.10.
192.168.1.10
It maps the IP address back to a hostname.
It makes server1 an alias for another hostname.