We create one default gateway for each subnet (VLAN).

 The key idea is this:

We don't create several default gateways for one network. We create one default gateway for each subnet (VLAN).

Let's understand why.

Example 1: One VLAN

Suppose your office has only one VLAN.

VLAN 10
Network: 192.168.10.0/24

PC1 192.168.10.10
PC2 192.168.10.20
Gateway: 192.168.10.1

Here, one default gateway is enough because all devices are in the same subnet.

Example 2: Three VLANs

Now the company grows.

VLAN 10 - Employees
192.168.10.0/24

VLAN 20 - Servers
192.168.20.0/24

VLAN 30 - CCTV
192.168.30.0/24

These are three different IP networks.

Each network needs a router interface (or SVI) inside that subnet.

VLAN 10 → Gateway 192.168.10.1
VLAN 20 → Gateway 192.168.20.1
VLAN 30 → Gateway 192.168.30.1

That's why you have three default gateways.

Why can't all VLANs use one gateway?

Suppose every PC uses:

192.168.10.1

as its gateway.

A PC in VLAN 20 has:

IP Address
192.168.20.10

Gateway
192.168.10.1

This won't work because:

  • The PC is on 192.168.20.0/24
  • The gateway is on 192.168.10.0/24

The PC cannot directly reach a gateway that is in a different subnet.

Real Enterprise Example

                  Layer 3 Switch

VLAN 10
Gateway 192.168.10.1

VLAN 20
Gateway 192.168.20.1

VLAN 30
Gateway 192.168.30.1

VLAN 40
Gateway 192.168.40.1

Each gateway is simply an interface on the same Layer 3 switch.

Think of it like an apartment building

Imagine:

Floor 1 = VLAN 10
Floor 2 = VLAN 20
Floor 3 = VLAN 30

Each floor has its own elevator entrance.

  • Floor 1 entrance = Gateway for VLAN 10
  • Floor 2 entrance = Gateway for VLAN 20
  • Floor 3 entrance = Gateway for VLAN 30

There isn't one entrance that every floor can directly use.

Connection to HSRP

With HSRP, each VLAN usually has its own virtual gateway.

Example:

VLAN 10 → Virtual IP 192.168.10.1
VLAN 20 → Virtual IP 192.168.20.1
VLAN 30 → Virtual IP 192.168.30.1

The PCs still have only one configured default gateway each:

  • A PC in VLAN 10 uses 192.168.10.1.
  • A server in VLAN 20 uses 192.168.20.1.
  • A camera in VLAN 30 uses 192.168.30.1.

They do not use all of the gateways.

The most important point

A common misconception is:

"If there are 10 default gateways in the network, every PC uses all 10."

That's not true.

  • Each device has exactly one default gateway configured.
  • The network has many default gateways because it has many different VLANs/subnets.
  • Each gateway serves only the devices in its own subnet.

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