How is the hostname for the IP address 198.51.100.165 stored on a DNS server?

The hostname for the IP address 198.51.100.165 is stored on a DNS server in a PTR record, which is specifically used for reverse DNS lookups. When you perform a reverse lookup, you need to provide the IP address in a specific format, which involves reversing the octets of the IP address and appending the suffix ".in-addr.arpa".

In this case, the IP address 198.51.100.165 would be referred to in reverse as 165.100.51.198.in-addr.arpa. The PTR record holds the hostname associated with that IP address, allowing DNS queries to map an IP back to a human-readable hostname. This is crucial for various applications, including email servers that check the PTR record to verify the identity of a connecting host.

Other DNS records serve different purposes: A records map hostnames to IP addresses, MX records are used for mail exchange servers and define where emails should be delivered, and CNAME records create aliases for other domain names but do not specifically relate to reverse lookups. Therefore, the association of the hostname with the given IP address is correctly facilitated through the PTR record in the reverse DNS.