Can the DNS system perform reverse lookups to convert an IP address to a domain name?

• A. It can convert an IP address to a DNS name
• B. It cannot map IPs back to names
• C. It requires a separate protocol
• D. It is unrelated to DNS

Answer: (A)

Reverse DNS lookups are a built-in feature of DNS, using PTR records to map an IP address back to a domain name. For IPv4, this works in the in-addr.arpa space by reversing the octets of the IP (for example, 203.0.113.5 becomes 5.113.0.203.in-addr.arpa) and attaching a PTR record that points to the domain name. For IPv6, the process uses the ip6.arpa space with nibble-by-nibble reversal. The actual lookup uses the PTR record type within DNS, so no separate protocol is needed. In practice, whether a reverse mapping exists depends on whether the administrator has configured a PTR record for that IP; many servers (notably mail servers) have one, but some IPs do not. So, DNS can convert an IP address to a domain name via reverse lookups.