Subnet Mask

Before understanding subnet masks, remember this:

An IP address has:

• Network part
• Host part

The subnet mask tells the computer where that split is.

What does a Subnet Mask do?

A subnet mask tells which part of the IP address represents the network.

Example:

IP Address  : 192.168.1.10
Subnet Mask : 255.255.255.0

This usually means:

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

So devices from 192.168.1.1 to 192.168.1.254 are usually in the same local network.

Why is it called a Mask?

Because it hides the host part and keeps the network part visible.

Think of the mask as saying:

255 → this part belongs to the network
0   → this part belongs to the host

For:

255.255.255.0

the first three octets are the network part.

The last octet is the host part.

Same Network Example

Device A
IP:   192.168.1.10
Mask: 255.255.255.0

Device B
IP:   192.168.1.20
Mask: 255.255.255.0

Both share:

192.168.1

So they are on the same network.

Different Network Example

Device A: 192.168.1.10
Device B: 192.168.2.20
Mask    : 255.255.255.0

Device A belongs to:

192.168.1.0

Device B belongs to:

192.168.2.0

They are on different networks.

To communicate, traffic must go through a router.

Interview Answer

If an interviewer asks:

What is a subnet mask?

You can answer:

A subnet mask tells a device which part of an IP address is the network part and which part is the host part. It is used to decide whether a destination IP is inside the local network or must be sent to a router.