Final answer:
Reverse DNS lookup is the process that enables a system to determine an FQDN by knowing the IP address. It is used by services like email providers to verify the authenticity of communication and is an essential component of Internet infrastructure.
Step-by-step explanation:
The process that enables a system to determine an FQDN (Fully Qualified Domain Name) by knowing the IP address is known as Reverse DNS lookup or rDNS. When a system has an IP address and needs to find out the associated domain name, this service uses a part of the DNS (Domain Name Service) to look up the domain name associated with that IP address. It's essentially the opposite of the more commonly known DNS lookup, where a domain name is translated to an IP address.
Reverse DNS lookups are important for services like email servers, where they are used to verify that the server sending the email matches the domain name it claims to be from. This helps to reduce spam and fraudulent emails. Essentially, if you have the IP address 192.0.2.1, a reverse DNS lookup would reveal the domain name associated with that particular IP address, such as example.com.