Internet architecture principles: which statement is incorrect regarding routing knowledge, peer-layer delivery, and address usage across layers?

Introduction:
The Internet stack separates concerns across layers: link-level physical/MAC addressing, network-level logical addressing, transport ports, and application names. This question asks you to spot the one statement that is incorrect by carefully examining how addressing is actually used across layers and how routing knowledge is distributed.

Given Data / Assumptions:

• Routers exchange and maintain routing information; hosts typically use default gateways and limited route knowledge.
• Peer layers communicate abstract “objects” (service data units) that are reconstructed at the destination.
• Upper layers use more than just IP addresses (e.g., ports, names, URIs); link layers handle physical addresses.

Concept / Approach:
Evaluate each claim. It is reasonable to assume routers know routes and hosts start with minimal information (default gateway, on-link routes). Layering is designed so layer-n at the destination reassembles the same layer-n object (modulo permitted transformations like compression/encryption). However, asserting that layers at and above the Internet layer use only IP addresses is too strong: applications primarily use domain names/URLs; transport relies on ports plus IP; security may use identities/certificates. Therefore option C is the incorrect overgeneralization.

Step-by-Step Solution:

Verification / Alternative check:
Everyday practice: users connect via DNS names, not raw IPs; transport endpoints are 4- or 6-tuples including IPs and port numbers; link layers still map to MAC addresses via ARP/ND, confirming the layered split.

Why Other Options Are Wrong (as incorrect choices):

• A reflects common host/gateway behavior.
• B expresses the essence of peer-layer delivery.
• D/E are distractors; exactly one statement (C) is incorrect.

Common Pitfalls:
Accepting “only IP addresses” at upper layers; forgetting the role of ports (Transport) and names (Application).

Final Answer:
Option C is incorrect because upper layers use more than just IP addresses (e.g., ports, DNS names).