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For example, to do a reverse lookup of the IP address 8.8.4.4 the PTR record for the domain name 4.4.8.8.in-addr.arpa would be looked up, and found to point to dns.google. If the A record for dns.google in turn pointed back to 8.8.4.4 then it would be said to be forward-confirmed .
These servers are registered in the DNS directory as the authoritative servers for the reverse lookup zone of the 10.0.0.0 / 8, 172.16.0.0 / 12 and 192.168.0.0 / 16 addresses. These servers are configured to answer any query with a "nonexistent address" answer.
Used for link-local addresses [5] between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from a DHCP server 172.16.0.0/12 172.16.0.0–172.31.255.255 1 048 576: Private network Used for local communications within a private network [3] 192.0.0.0/24 192.0.0.0–192.0.0.255 256
A reverse DNS lookup is a query of the DNS for domain names when the IP address is known. Multiple domain names may be associated with an IP address. The DNS stores IP addresses in the form of domain names as specially formatted names in pointer (PTR) records within the infrastructure top-level domain arpa. For IPv4, the domain is in-addr.arpa.
In practice, this meant that an HTTPS server could only serve one domain (or small group of domains) per IP address for secured and efficient browsing. Assigning a separate IP address for each site increases the cost of hosting, since requests for IP addresses must be justified to the regional Internet registry and IPv4 addresses are now exhausted.
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The IP address of a public server is also important, similar in global uniqueness to a postal address or telephone number. Both IP address and port number must be correctly known by all hosts wishing to successfully communicate. Private IP addresses as described in RFC 1918 are usable only on private networks not directly connected to the internet.
The HTTP specification suggests the phrase "Not Found" [1] and many web servers by default issue an HTML page that includes both the 404 code and the "Not Found" phrase.