Why is the network layer considered "routable"?

The network layer is considered "routable" because it is exactly at this layer that the decision is made about which networks and devices to send data through, and it is here that delivering data between different networks becomes possible.

Short and clear: only the network layer can direct data beyond a single local network.

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#What "routable" means

"Routable" means that data:

• can be transmitted through several networks
• passes through routers
• at each stage receives a decision:
  • where to send the packet next

This is possible only when logical addressing and routes exist.

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#Why this is possible specifically at the network layer

#1. The presence of logical addresses (IP)

The network layer uses IP addresses, which:

• show network membership
• allow distinguishing "your own" network from "someone else's"
• are understood identically by all routers

MAC addresses do not work this way:

• they are local
• they carry no information about the network's structure

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#2. How routers work

Routers:

• operate at the network layer (L3)
• analyze the IP header of the packet
• use routing tables
• choose the next hop for forwarding the packet

Without the network layer, routers would be impossible.

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#3. Independence from the data link layer

The network layer:

• does not depend on the L2 technology (Ethernet, Wi-Fi, and so on)
• can carry packets over different types of networks

This allows a packet to:

• pass through different data link technologies
• not "break" along the way

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#4. A single packet format

An IP packet:

• is preserved from source to destination
• is used by all routers
• is forwarded regardless of the transmission medium

Only these change:

• data link layer frames
• the physical medium

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#What is important for an interview

• routing = L3
• packets are routed, not frames
• IP is a routable protocol
• MAC is a non-routable address

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#In short, for an interview

The network layer is considered routable because it uses logical IP addresses and supports route selection and the transmission of packets between different networks through routers.