Basic Network Address Translation or Basic NAT is a method by which IP addresses are mapped from one group to another, transparent to end users.
The need for IP Address translation arises when a network's internal IP addresses cannot be used outside the network either for privacy reasons, or because they are invalid for use outside the network.
Network topology outside a local domain can change in many ways. Customers may change providers, company backbones may be reorganized, or providers may merge or split. Whenever external topology changes with time, address assignment for nodes within the local domain must also change to reflect the external changes. Changes of this type can be hidden from users within the domain by centralizing changes to a single address translation router.
See also RFC 3022 NAT-TERM https://tools.ietf.org/html/rfc3022#ref-NAT-TERM