What is a subnet mask?

A subnet mask is a parameter that determines which part of an IPv4 address belongs to the network and which belongs to the hosts within it.

Short version for an interview:

A subnet mask is a 32-bit number that separates the network part of an IP address from the host part. Thanks to the mask, a router understands which network this is and which devices are in it.

---

#What a subnet mask looks like

A mask has the same format as an IPv4 address: four numbers from 0 to 255.

Examples of common masks:

But the shorthand CIDR form is used more often:

---

#The meaning of the subnet mask

The mask determines:

• which bits belong to the network → they are set to 1
• which bits belong to the hosts → they are set to 0

Example of the 255.255.255.0 (/24) mask in binary form:

11111111.11111111.11111111.00000000

First 24 bits → network Last 8 bits → hosts

---

#Example of how a subnet mask works

Suppose there is an IP:

192.168.10.25

And a mask:

255.255.255.0 (/24)

This means:

• Network part: 192.168.10
• Host part: 25

Network:

192.168.10.0/24

Broadcast:

192.168.10.255

Host range:

192.168.10.1 - 192.168.10.254

---

#Why a subnet mask is needed

#1. To determine subnet boundaries

The router understands:

• which IPs belong to the same network,
• where to send packets,
• where routing is required.

#2. To divide a network into subnets

For example, a /24 network can be split into two /25s:

• 192.168.1.0/25
• 192.168.1.128/25

#3. To optimize routing

CIDR and VLSM allow routing tables to be reduced.

---

#An important formula

Number of hosts in a network:

2^(number of zero bits in the mask) - 2

Examples:

---

#Common masks and their applications

---

#Ready answer for an interview

A subnet mask is a 32-bit number showing which part of an IPv4 address belongs to the network and which belongs to the hosts. In the mask, ones denote the network part, zeros denote the host part. The mask allows you to determine subnet boundaries, route traffic, and divide a network into smaller subnets. Example: for the 255.255.255.0 (/24) mask, the network is the first 24 bits, the hosts are the last 8.

---

#Cheat sheet (5 seconds)

• Subnet mask = separates Network from Host.
• In the mask: 1 = network, 0 = hosts.
• Notation: 255.255.255.0 = /24.
• Determines: network, broadcast, host range.
• Hosts = 2ⁿ - 2, where n = number of zeros.