In legacy X.25 addressing (Network User Address, NUA), which digit positions are commonly treated as the “area code” within the national portion of the address?

Introduction / Context: X.25 networks used the X.121 numbering plan. Subscriber destinations were identified by a Network User Address (NUA). Understanding how the digits are structured—international versus national parts—helps interpret legacy addressing and routing on packet-switched public data networks.

Given Data / Assumptions:

• NUA follows the X.121 format up to 14 decimal digits.
• Digits 1–4 are the DNIC (country + network code).
• The “area code” referenced is within the national (post-DNIC) portion.
• We select the digit positions typically used as the area indicator.

Concept / Approach: In X.121, the first four digits (DNIC) identify the country and public data network. The remaining digits comprise the national terminal number (NTN). A common convention in many networks was to use digits 5–7 (the first three digits of the NTN) as an “area” or regional code, followed by subscriber/location specifics. While countries could define different NTN structures, “5–7” is the canonical exam convention for the area portion after the DNIC.

Step-by-Step Solution: Partition address: DNIC = digits 1–4; NTN begins at digit 5.Identify the area subfield within the NTN: digits 5–7 in many national plans.Therefore select “5–7” for the area code positions.

Verification / Alternative check: Many historical carrier documents describe DNIC (1–4) then national structure as area (5–7) + subscriber (8–12) + optional extensions. Differences existed across countries, but standardized training and exams typically adopt this mapping for consistency.

Why Other Options Are Wrong: 1–4: DNIC (international portion), not the national “area.”

8–12: Often subscriber or site digits, not the area.

13–14: Typically extension/special-purpose digits, not area.

None of the above: Incorrect because 5–7 is the conventional answer.

Common Pitfalls: Assuming all countries used identical NTN layouts; remember that 5–7 is the standard exam model even if national implementations varied.

Final Answer: 5–7