IS-IS NET Structure: Area Address Components
Short answer
AC The Area Address part of an IS-IS NET consists of the AFI, IDI, and High Order DSP, while the System ID is a separate field.
Question
Key Takeaways
Test-takers often confuse the entire NET with just its parts; remember that System ID is distinct from the Area Address.
In IS-IS networking, the Network Entity Title (NET) is divided into Area Address, System ID, and N-Selector. The Area Address specifically contains the IDI and High Order DSP.
Common mistake: Selecting D (System ID) is a common error because it is part of the NET, but it is not part of the Area Address field.
Expert Analysis
The Network Entity Title (NET) in IS-IS is a 12-14 byte address structured into three distinct fields: Area Address, System ID, and N-Selector. To determine the correct options, we must examine the composition of the Area Address field specifically.
The Area Address itself is composed of:
- AFI (Authority and Format Identifier): The first byte, typically
0x39for IPv4 Internet addresses. - IDI (Initial Domain Identifier): Identifies the administrative domain.
- High Order DSP (Designated System Part): Used to further subdivide the area; it is optional but part of the Area Address structure when present.
Let's evaluate the options against this structure:
- A. High Order DSP: Correct. As noted in the comments, the Area Address includes AFI, IDI, and optionally the High Order DSP.
- B. API: Incorrect. This appears to be a distractor or typo for AFI, but 'API' is not a standard field name in the NET structure.
- C. IDI: Correct. The Initial Domain Identifier is a core component of the Area Address.
- D. System ID: Incorrect. The System ID is a separate field from the Area Address. It follows the Area Address and uniquely identifies the router within that area.
Therefore, the fields contained within the Area Address part are AFI (implied/related), IDI, and High Order DSP. Since AFI is not an option but IDI and High Order DSP are, these are the correct selections.
Exam Strategy
When asked about protocol headers or addressing structures, break them down into their constituent bytes or fields. For IS-IS NET, visualize the 12-14 byte string as [Area Address][System ID][N-Selector]. Always verify if the question asks for the whole entity or a specific sub-component like the Area Address.
Frequently Asked Questions
What is the total length of an IS-IS NET?
What does the N-Selector in IS-IS stand for?
Community Votes
50% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 2 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 2
-
198a491 👍 1No reference to API and AFI as same in the official material
-
HCIP Resolutor 👍 1{Area Address} = {AFI} + {IDI} + {High Order DSP}
Reader comments & corrections
No reader corrections yet — spotted an error or have a note? Share it below.
Related Analysis
Practice All H12-821 Questions
Access 168 questions with complete answers and detailed explanations.
View Full H12-821 Practice Test →