Reverse DNS turns an IP address into a hostname -- for example, it might turn 192.0.2.25 into host.example.com. Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses. One of the applications of reverse DNS is as a spam filter.
Because IP addresses are tedious to deal with and hard to remember, an IP address may also be assigned a host or domain name. You can think of an IP address as someone's physical street address and the host/domain as Bob's House. Multiple host/domain names can be directed to one IP address (as is often the case and referred to as virtual hosting). Host names may be looked up to find IP addresses, and visa-versa.
In common usage today, hostname refers specifically to the FQDN that comprises the name of the host, a period, and the domain name. The domain name is composed of labels that begin with the top-level domain and are arranged right to left for the second and, possibly, third level labels. The host name and the domain labels are separated by periods. The total length of the hostname as an FQDN cannot exceed 255 character with each domain name label having a maximum length of 63 characters. For example, if a company’s domain name is “example.com.” and a particular computer in the company has been named “plato”, the hostname for that computer will be “plato.example.com.”. Note, that technically FQDNs end in a period to unambiguously define the hostname and to be sure that it is not part of a longer domain name. (although in common usage this final period is often omitted).
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