11.3 Reverse Domains

We have already said that communication between hosts is based on IP addresses, not domain names. On the other hand, some applications need to find a name for an IP address—in other words, find the reverse record. This process is the translation of an IP address into a domain name, which is often called reverse translation.

As with domains, IP addresses also create a tree structure (see Figure 11.2). Domains created by IP addresses are often called reverse domains. The pseudodomains IP6.arpa for IPv6 and in-addr.arpa for IPv4 were created for the purpose of reverse translation. The latter domain name has historical origins; it is an acronym for inverse addresses in the Arpanet.

Under the domain in-addr.arpa, there are domains with the same name as the first number from the network IP address. Thus the in-addr.arpa domain has subdomains 0 to 255. Each of these subdomains also contains lower subdomains 0 to 255. For example, network 195.47.37.0/24 belongs to subdomain...