Below is a description of Network Tool "DNS Lookup" that also describes Network Tool "Reverse DNS Lookup".

"DNS Lookup" takes a Domain Name and provides its IPv4 address and any registered IPv6 address. ("DNS Lookup" was used above to find Google's IPv4 and IPv6 addresses.) This is known as a forward DNS. There can be multiple IP addresses associated with the Domain Name.

1. Example of "DNS Lookup"
Input "yahoo.com" and click "Submit"

Output:
Server: 64.20.34.50
Address: 64.20.34.50#53
Non-authoritative answer:
Name: yahoo.com
Address: 74.6.231.20
Name: yahoo.com
Address: 98.137.11.163

"Reverse DNS Lookup" takes an IP address and provides it Domain Name.

2. Example of "Reverse DNS Lookup"

Input the first yahoo.com server in example 1 above "64.20.34.50" and click "Submit"

Output: IPv4/IPv6 information lookup
50.34.20.64.in-addr.arpa name = cache2.trouble-free.net.
50.34.20.64 is the server 64.20.34.50 in reverse order. "50.34.20.64.in-addr.arpa" is the filename of the PTR (Pointer) record stored on the server 64.20.34.50 that contains its domain name.

3. Example of "DNS Lookup"

Input the domain name "cache2.trouble-free.net" from Example 2 and click "Submit"

Output:
Server: 64.20.34.50
Address: 64.20.34.50#53

server IP address 64.20.34.50 in example 3 is the same as the server IP address input in Example 2.

We now have a Forward-confirmed reverse DNS (FCrDNS) also known as full-circle reverse DNS.

DNS "cache2.trouble-free.net" -> "64.20.34.50"
rDNS "64.20.34.50" -> "cache2.trouble-freenet"

DNS doesn't require Reverse DNS records, and forward and reverse DNS records do not even have to agree with each other (See Example 2) - but if they do, then we have a FCrDNS, which is a weak form of authentication and is becoming required by mail servers for verification.