Packet Traversals of Network and Data Link Layers Across Two Intermediate Routers

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Nov 4, 2026

Consider a path where source $S$ sends to destination $D$ through two intermediate routers labeled $R_1$ and $R_2$ , i.e., the hop sequence is: $S \rightarrow R_1 \rightarrow R_2 \rightarrow D$ .

At each router hop, a packet must be processed by:

the network layer (routing/forwarding; e.g., IP forwarding), and
the data link layer (encapsulation for transmission on that specific link).

In general, for store-and-forward packet switching, the packet traverses the network layer at every node where it is forwarded, and it traverses the data link layer once per physical/logical link transmission on the end-to-end path. This corresponds to counting hops on the $S \to D$ route.

Key terms used below:

network layer
data link layer
hop
router forwarding

OSI Model: Network Layer vs Data Link Layer (Hop-by-hop intuition)

1) How many times does each packet visit the network layer?

For $S \rightarrow R_1 \rightarrow R_2 \rightarrow D$ :

The packet is delivered to the network layer at:

Source $S$ (network-layer creates/starts the packet for routing)
Router $R_1$ (network-layer forwarding decision)
Router $R_2$ (network-layer forwarding decision)
Destination $D$ (network-layer receives/delivers to the upper layer)

So the network-layer visits are:

\text{Network-layer visits} = 4

This matches the idea: # network-layer visits = (# routers + 2 endpoints).

2) How many times does each packet visit the data link layer?

Data link layer processing happens for each link transmission along the route:

Links traversed are:

Link $S \rightarrow R_1$
Link $R_1 \rightarrow R_2$
Link $R_2 \rightarrow D$

That is 3 links, hence 3 data link layer visits (one per hop transmission).

So the data-link-layer visits are:

\text{Data-link-layer visits} = 3

This corresponds to: # data-link-layer visits = # hops.

type="tip" title="Counting rule" content="If the path is $S \rightarrow R_1 \rightarrow \cdots \rightarrow R_k \rightarrow D$ , then: Network-layer visits = $(k+2)$ and data-link-layer visits = $(k+1)$ ."

type="warning" title="Potential ambiguity to watch for" content="Some courses count “visits” differently (e.g., excluding source-side encapsulation or excluding destination delivery). The standard hop-by-hop interpretation counts network-layer at every node along the path and data-link-layer once per link transmission."

Step-by-step packet-layer traversal for S → R1 → R2 → D

1
Step 1
The route has 3 hops: $S\to R_1$ , $R_1\to R_2$ , and $R_2\to D$ .
2
Step 2
Each hop uses the data link layer to frame and transmit on that link. With 3 hops, data link layer visits = 3.
3
Step 3
The network layer is involved at source, each router, and the destination: $S$ , $R_1$ , $R_2$ , $D$ → 4 nodes → network layer visits = 4.
4
Step 4
Network layer visits: 4. Data link layer visits: 3.

Layer Visits for S → R1 → R2 → D

Counts per packet along the route with two intermediate routers.

Hop-by-hop perspective of layer involvement

Data link: frame & send

1st hop (S → R1)

Data link layer processes the transmission over the first link."

Network: forward

At R1

Network layer determines the next hop toward $R_2$ ."

Data link: frame & send

2nd hop (R1 → R2)

Data link layer processes the transmission over the second link."

Network: forward

At R2

Network layer determines the next hop toward $D$ ."

Data link: frame & send; Network: deliver

3rd hop (R2 → D)

After arrival at $D$ , the network layer delivers to the destination application layer."

Common variants & interpretation

Quick self-check flashcards

1 / 4

Question · Term

For S → R1 → R2 → D, what is the number of hops?

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Answer · Definition

3 hops: $S\to R_1$ , $R_1\to R_2$ , $R_2\to D$ .

Knowledge Check

Question 1 of 4

Q1Single choice

For the route $S \rightarrow R_1 \rightarrow R_2 \rightarrow D$ , how many times does the packet visit the network layer?

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