As each domain is further divided into sub-domains, each becoming a DNS zone itself with its own set of administrators and DNS servers, the tree grows with the largest number of leaf nodes at the bottom. At this lowest level, in the end-nodes or leaves of the tree, the term DNS zone becomes essentially synonymous with the term "domain", both in terms of use and administration. The term domain is used in the business functions of the entity assigned to it and the term zone is usually used for configuration of DNS services. Forward DNS zones DNS zones contain the records for the mapping of domain names to IP addresses or other information. The resolution of a domain name to its assigned information is also referred to as forward resolution and the DNS zones associated with such processes are often referred to as forward zones. The term arose as the opposite of reverse zones, which are used for the reverse process: finding the DNS name associated with an IP address. Such reverse zones are maintained in the Internet Address and Routing Parameter Area (domain arpa). Another common use of the term forward zone refers to a specific configuration of DNS name servers, particularly caching name servers, in which resolution of a domain name is forwarded to another name server that is authoritative for the domain in question, rather than being answered from the established cache memory. Zones for Internet infrastructure The top-level domain arpa serves as a delegation zone for various technical infrastructure aspects of DNS and the Internet, and does not implement the registration and delegation system of the country and generic domains. The name arpa is a remnant of the ARPANET, one of the predecessor stages of the Internet. Intended as a transitional aid to the DNS, deleting the domain arpa was later found to be impractical. Consequently, the name was officially redefined as an acronym for Address and Routing Parameter Area. It contains sub-zones used for reverse resolution of IP addresses to host names (IPv4: in-addr.arpa, IPv6: ip6.arpa), telephone number mapping (ENUM, e164.arpa), and uniform resource identifier resolution (uri.arpa, urn.arpa). 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