NXXN

NXXN is a four-character numeric pattern used in telecommunications, computing, and documentation as a placeholder for digit constraints. In this notation, N represents a non-zero digit, typically the digits 2 through 9, while X represents any digit from 0 through 9. The combination NXXN therefore denotes a four-digit sequence in which the first and last digits must be non-zero, and the middle two digits may be any digits.

In the context of the North American Numbering Plan (NANP), numbers are written in the form NPA-NXX-XXXX, where N is 2–9 and X is 0–9. The middle three digits (the NXX block) designate the central office or exchange. While 'NXXN' is not an official term in NANP documentation, it is sometimes used in informal discussion or documentation to refer to a four-digit fragment of a phone number that starts with a non-zero digit, has two variable digits in the middle, and ends with a non-zero digit. It may be used to illustrate routing rules, validation checks, or template-based generation of sample numbers.

Outside telephony, NXXN can appear in generic templates for form validation, database keys, or programming exercises where digit-level constraints are illustrated. There is no widely recognized entity, organization, or standard known simply as 'NXXN.' The term is context-dependent and typically defined by accompanying explanations.