Classless Inter-Domain Routing
An IP address is interpreted as composed of two parts: a network-identifying prefix followed by a host identifier within that network. In the previous classful network architecture, IP address allocations were based on the bit boundaries of the four octets of an IP address. An address was considered to be the combination of an 8, 16, or 24-bit network prefix along with a 24, 16, or 8-bit host identifier respectively. Thus, the smallest allocation and routing block contained only 256 addresses—too small for most enterprises, and the next larger block contained addresses—too large to be used efficiently even by large organizations. This led to inefficiencies in address use as well as inefficiencies in routing, because it required a large number of allocated class-C networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation.
DNS zone
Although the administrative structure of this domain and its sub-domains is different, the technical delegation into zones of responsibility is similar and the DNS tools and servers used are identical to any other zone. Sub-zones are delegated by components of the respective resources. For example, 8.8.2.5.5.2.2.0.0.8.1.e164.arpa., which might represent an E.164 telephone number in the ENUM system, might be sub-delegated at suitable boundaries of the name. An example of an IP addresses in the reverse DNS zone is 166.188.77.208.in-addr.arpa, which represents the address 208.77.188.166 and resolves to the domain name "www.example.com". In the case of IP addresses, the reverse zones are delegated to the Internet service provider (ISP) to which the IP address block is assigned. When an ISP allocates a range to a customer, it usually also delegates the management of that space to the customer by insertion of name server resource records pointing to the customer's DNS facilities into their zone, or provides other management tools. Allocations of single IP addresses for networks connected through network address translation (NAT) typically do not provide such facilities.