Reverse DNS Lookup (PTR Records)

Reverse DNS, or rDNS, is the opposite of regular DNS. Instead of asking what IP a hostname resolves to, you ask what hostname an IP claims to be. The answer comes from a PTR (pointer) record in a special reverse zone managed by the IP block owner. Reverse DNS is most commonly used by mail servers, which often refuse mail from sending IPs without a valid PTR. It's also a quick way to identify whether an IP belongs to a residential ISP, a hosting provider, or a known service. Many residential IPs have generic PTR strings like `pool-23-45-67.example-isp.net`, while hosting IPs typically show the provider's domain.