In the X.121 addressing scheme for public data networks, how many digits of the Data Network Identification Code (DNIC) represent the country (Data Country Code)?

Introduction:
The X.121 addressing plan was used in public packet-switched data networks (such as older X.25 services). At the core of X.121 is the Data Network Identification Code (DNIC), which identifies both the country and the specific public data network. This question probes whether you know how many leading digits of the DNIC designate the country itself.

Given Data / Assumptions:

• Addressing standard: X.121 for public data networks.
• DNIC length: 4 digits total.
• We must determine how many of those four digits are assigned to the country code portion.

Concept / Approach:

Within the DNIC, the first part is the Data Country Code (DCC), and the remaining part identifies the public data network within that country. Recognizing this split allows you to pinpoint the number of digits tied specifically to the country.

Step-by-Step Solution:

Verification / Alternative check:

Example patterns and standards documentation consistently show DNIC = DCC (3 digits) + Network (1 digit). Hence, the country is encoded in the first three digits, verifying the answer.

Why Other Options Are Wrong:

First four digits: That would include the network identifier, not just the country.

First five digits: DNIC has only four digits; five is impossible.

First six digits: Exceeds DNIC length and conflates DNIC with the remainder of the X.121 address.

None of the above: Incorrect because 'first three digits' is standard.

Common Pitfalls:

Confusing the entire X.121 address (which can be much longer) with the DNIC subfield, and assuming all leading digits indicate country. Only the first three digits of the DNIC are the country code; the fourth digit is the network code.

Final Answer:

First three digits