X.121 addressing (NUA): within a Network User Address, which digit positions are reserved for the user’s sub-address digits?

Introduction / Context:
Public packet-switched data networks that followed CCITT/ITU-T X.25 used X.121 addressing. A Network User Address (NUA) contains a country/network identifier segment and a national terminal number segment, with certain trailing digits sometimes designated for user sub-addressing purposes (for example, to reach a specific application mailbox or port on the destination system).

Given Data / Assumptions:

• We are selecting digit positions used as user sub-address digits.
• The NUA length may extend up to 14 digits in common practice.
• Early digits identify the network (DNIC), later digits identify the user endpoint and sub-address.

Concept / Approach:
In a typical X.121 NUA structure, digits 1–4 are the Data Network Identification Code (DNIC). Subsequent digits form the national terminal number. The last two digits (positions 13–14 when a 14-digit NUA is used) are often treated as user-controlled sub-address digits, enabling further selection at the subscriber’s site.

Step-by-Step Solution:

Verification / Alternative check:
Vendor documentation and historical NUA plans frequently describe the final two digits as user-selected sub-addresses, distinct from the carrier-assigned network and subscriber portions.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming a shorter NUA length and forgetting the special role of the final digits; confusing national numbering with sub-address space.

Final Answer:
13-14.