In the four-layer DoD (TCP/IP) model, which layer corresponds functionally to the Network layer of the OSI model?

Introduction / Context:
The Department of Defense (DoD) model, also called the TCP or IP model, is a four-layer framework that predates but aligns roughly with the seven-layer OSI model. Exam questions often ask you to map specific OSI layers to their approximate counterparts in the DoD model.

Given Data / Assumptions:

• The DoD or TCP or IP model layers are Application, Host-to-Host, Internet, and Network Access.
• The OSI Network layer (Layer 3) is responsible for logical addressing and routing.
• We need the DoD layer that performs similar functions.

Concept / Approach:
In the OSI model, the Network layer handles logical addressing (for example, IP addresses), path selection, and routing between networks. In the DoD model, the Internet layer plays the same role, hosting protocols such as IP, ICMP, and routing protocols. The Host-to-Host layer corresponds more closely to the OSI Transport layer, and the Network Access layer combines OSI Data Link and Physical layer functions.

Step-by-Step Solution:
Step 1: Recall that the DoD Application layer roughly covers OSI layers 5 to 7, dealing with application services.Step 2: Recognize that the DoD Host-to-Host layer is where TCP and UDP operate, providing end-to-end transport, similar to OSI Layer 4.Step 3: The DoD Internet layer is where IP addressing and routing occur, which is exactly what the OSI Network layer does.Step 4: The DoD Network Access layer handles physical media and link layer functions, equivalent to OSI Layers 1 and 2.

Verification / Alternative check:
Standard mapping diagrams show IP and routing protocols in the Internet layer and describe it as responsible for addressing and routing. This is the same functionality associated with OSI Layer 3, confirming that the Internet layer is the correct match.

Why Other Options Are Wrong:
Option A, the Application layer, corresponds to OSI layers 5 through 7 rather than the Network layer.Option B, the Host-to-Host layer, aligns with the OSI Transport layer, not the Network layer.Option D, the Network Access layer, aligns with OSI Data Link and Physical layers, not with logical addressing and routing.

Common Pitfalls:
Some learners confuse the Internet layer with the Application layer because both sound high level. Remember that the key function of the OSI Network layer is routing based on logical addresses, and that function clearly lives in the Internet layer of the DoD model.

Final Answer:
The DoD layer equivalent to the OSI Network layer is the Internet layer.