X.121 addressing (NUA): in a full 14-digit Network User Address, which digit positions constitute the terminal (national) number portion, excluding the DNIC and the optional user sub-address?

Introduction / Context: X.25 public data networks used the ITU-T X.121 numbering plan. An X.121 Network User Address (NUA) commonly has up to 14 digits: the first four digits are the Data Network Identification Code (DNIC), the next block is the national (terminal) number, and, in many implementations, the last two digits can serve as a user-level sub-address. Understanding which positions represent the terminal number is a frequent standards question.

Given Data / Assumptions:

• We consider a 14-digit NUA.
• Digits 1–4 are the DNIC (country/network identifier).
• Digits 13–14 are often treated as user sub-address digits.
• The terminal number occupies the remaining middle digits.

Concept / Approach: With 14 digits total, and DNIC = 1–4 and sub-address = 13–14, the national terminal number spans digits 5 through 12 (eight digits). These identify the subscriber endpoint within the national network numbering plan.

Step-by-Step Solution:

Verification / Alternative check: Historical NUA formats and operator documentation describe DNIC (4 digits) followed by national number (typically 8 digits) and sometimes user sub-address, matching positions 5–12 for the terminal number.

Why Other Options Are Wrong:

Common Pitfalls: Forgetting that the total is 14 digits and that the last two are often reserved for sub-addressing; mixing national number length with country-specific variations—here we use the common 14-digit model.

Final Answer: 5-12.