Network-layer identification: which of the following is NOT an example of a network-layer protocol or function (choose the item that belongs to a different OSI layer)?

Introduction / Context:
The OSI model separates concerns across layers. The network layer (Layer 3) provides logical addressing and routing across multiple links, while the data link layer (Layer 2) ensures reliable transfer over a single link. This question asks you to pick the one item that is clearly not a network-layer protocol or function, emphasizing accurate mapping to OSI layers.

Given Data / Assumptions:

• IP is a canonical network-layer protocol.
• X.25 defines both a packet layer (network layer) and a link layer (LAPB).
• “Source routing” is a network-layer concept; “domain naming” belongs to applications, but the option pairs them in a historical context.

Concept / Approach:
Map each entry to the OSI layer it best represents. IP is firmly Layer 3. X.25 PLP (Packet Layer Protocol) implements virtual circuits at Layer 3. X.25 Level 2 (LAPB) is Link Access Procedure, Balanced—a Layer 2 (data link) protocol, not Layer 3. The “source routing and domain naming – Usenet” phrasing mixes layers, but source routing itself is a network-layer feature; the presence of an application concept in the same phrase does not change the fact that source routing maps to Layer 3. The one unambiguous non–Layer 3 entry is LAPB.

Step-by-Step Solution:

Verification / Alternative check:
Any protocol reference shows LAPB as ISO data link layer; PLP is the X.25 network layer. IP remains the Internet’s Layer 3 protocol, confirming the separation.

Why Other Options Are Wrong:

• IP: quintessential Layer 3.
• X.25 PLP: defined as the packet (network) layer of X.25.
• Source routing and domain naming: includes a Layer-3 concept (source routing); the item is not purely non-network-layer.
• None of the above: incorrect because one item is definitely not Layer 3.

Common Pitfalls:
Confusing X.25's layered components; assuming that application-layer naming invalidates the presence of a network-layer function; forgetting that LAPB is link-layer framing and error control only.

Final Answer:
X.25 Level 2 (LAPB) – ISO.