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; the client identifier in DHCP is the hostname hashed with CRC32. A DHCP relay agent converts unicast discovers into broadcasts so remote servers can hear them. Hosts renew leases by contacting any DHCP server, not the one that issued the lease, as the protocol keeps no server identity. DHCP snooping assigns leases faster by caching offers in the switch ASIC
• B.PTR records redirect HTTP traffic to HTTPS using a server-side 301 redirect
• C.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