What is 'reverse DNS lookup' using PTR records, and for what two purposes is it most commonly used in enterprise networks?

• A.Reverse DNS converts IPv6 addresses to IPv4 addresses using PTR records in the in-addr.arpa zone
• B.Reverse DNS lookup maps an IP address to a hostname using PTR records stored in a special reverse DNS zone (in-addr.arpa for IPv4, ip6.arpa for IPv6); common enterprise uses: (1) Email anti-spam -- receiving mail servers verify that the sender's IP has a matching PTR record pointing to the claimed sending hostname, rejecting mail that fails; (2) Network device monitoring/logging -- SNMP/syslog systems resolve IP addresses to hostnames for human-readable logs, making troubleshooting and audit review easier
• C.PTR records redirect HTTP traffic to HTTPS using a server-side 301 redirect
• D.Reverse DNS is used exclusively by DNSSEC to verify zone signing key ownership