In X.121 addressing for public data networks, how many leading digits of the Network User Address constitute the DNIC (Data Network Identification Code)?

Introduction:
The X.121 addressing scheme was used by X.25 public data networks to identify networks and subscribers. A critical element of the address is the DNIC, which directs packets to the correct national or international data network before final delivery to the subscriber endpoint.

Given Data / Assumptions:

• An X.121 Network User Address (NUA) begins with a network-identifying prefix.
• DNIC is a fixed-length portion at the start of the NUA.
• We seek the exact length of that prefix in digits.

Concept / Approach:
The DNIC uniquely identifies a public data network and is positioned at the head of the NUA. It is composed of four digits: a three-digit country/network zone code plus a single digit identifying the specific public data network within that country/zone. After the DNIC, the remaining digits identify the subscriber within that network.

Step-by-Step Solution:
1) Recognize the NUA structure: DNIC + subscriber number.2) Recall that DNIC length is standardized to 4 digits.3) Conclude that the first four digits of the NUA form the DNIC.

Verification / Alternative check:
Historic X.25 documentation and implementations consistently parse the first four digits to route traffic into the proper PDN before subscriber-level lookup.

Why Other Options Are Wrong:

• First three/five/seven: do not match the standardized 4-digit DNIC.
• None of the above: invalid because the correct standard length is defined.

Common Pitfalls:
Confusing country codes in telephony (E.164) with DNIC; although both are prefixes, their lengths and purposes differ.

Final Answer:
first four