Network ID & Host ID

An IP address has two logical parts:

Think of a home address:

City      → Network
House No. → Host

The city tells you the area.

The house number tells you the exact house inside that area.

Simple Example

Suppose two laptops are connected to the same Wi-Fi:

Laptop A: 192.168.1.10
Laptop B: 192.168.1.20

Both start with:

192.168.1

Only the last number changes.

In a common home network, you can think of it like this:

192.168.1 | 10
─────────   ──
 Network    Host

So:

Why do we need this split?

The split helps a device answer:

Is the destination on my local network, or do I need to send this to the router?

Example:

My IP        : 192.168.1.10
Destination : 192.168.1.20

Both are in the same network.

The device can send locally.

But:

My IP        : 192.168.1.10
Destination : 8.8.8.8

This is outside the local network.

The device sends it to the default gateway, usually your router.

Important Point

You cannot always tell the network and host parts just by looking at the IP address.

You need a subnet mask or CIDR notation to know the exact split.

Example:

192.168.1.10/24

Here /24 tells us how much of the address is the network part.

Interview Answer

If an interviewer asks:

What are Network ID and Host ID?

You can answer:

Network ID identifies the network a device belongs to. Host ID identifies the specific device inside that network. The subnet mask or CIDR prefix tells us where the IP address is split between network and host parts.