Reverse DNS lookup: how PTR records turn an IP address into a name

For IPv4, the four numbers are written backward under in-addr.arpa, so 192.0.2.10 becomes 10.2.0.192.in-addr.arpa. The order flips because DNS names get more specific to the left, while IP addresses get more specific to the right. For IPv6, every hex digit of the full address is written backward under ip6.arpa (RFC 3596). That makes a name with 32 digits, one per label.

The reverse zone belongs to whoever holds the address block, not to the owner of a domain. Regional registries like ARIN and RIPE NCC hand reverse zones to the networks they give addresses to, and those pass control on to their customers, or keep it. So you can't add a PTR record at your domain's DNS host. You set it at your hosting, cloud or internet provider, often in a reverse DNS field next to the server's IP address.

A PTR record on its own proves little, since the address holder can type in any name, including yours. That's why mail servers check both ways. They read the PTR name, look up that name's A or AAAA records, and expect the same address back. This is called forward confirmed reverse DNS. Gmail requires it of every sender, and a server that fails it gets error 550 5.7.25, which says the IP has no PTR record or its name doesn't point back.

Private addresses, like 192.168.x.x and 10.x.x.x (RFC 1918), have no names in public DNS, so public resolvers answer NXDOMAIN for them. Only the DNS server on that network, like a router or an Active Directory server, can know their names.

Type an IP address into the DNS Lookup and it asks for the PTR record, then looks up each name it finds to see if it points back. Try 8.8.8.8: its PTR record is dns.google, and dns.google points back to 8.8.8.8.