What is a subnet?

A subnet is a logically distinct part of an IP network, created by dividing one network into several smaller ones.

Short wording for an interview:

A subnet is a group of IP addresses united by one network part, defined by the subnet mask. All devices within a subnet can communicate with each other directly and are in the same broadcast domain.

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#The basic idea of subnets

A subnet consists of:

• a network address
• a range of usable hosts
• a broadcast address
• a subnet mask (/xx)

The boundary between the network and host parts is determined by the subnet mask.

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#A simple example

Subnet:

192.168.1.0/24

• Network: 192.168.1.0
• Broadcast: 192.168.1.255
• Host range: 192.168.1.1 - 192.168.1.254
• Number of hosts: 254

All these addresses are: one subnet.

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#What using subnets gives you

#1. Dividing the network into smaller segments

Allows splitting a large network into:

• departments (IT, accounting, HR)
• VLANs
• different branches
• server segments

This reduces load and makes the network more structured.

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#2. Limiting broadcast domains

Broadcast (ARP, DHCP) operates within a subnet. If a subnet is too large → a lot of broadcast traffic → load.

Subnets help isolate traffic, improving performance.

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#3. Increasing security

Different subnets can be filtered at the level of:

• ACLs
• a firewall
• a router

For example:

• servers in the 10.0.10.0/24 subnet
• users: 10.0.20.0/24
• access policies are applied between them

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#4. Efficient routing

Routers use the network part of the IP to determine where to send a packet.

Subnets:

• reduce the routing table
• are convenient for aggregation (summarization)

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#Subnet calculation formula

Number of hosts:

2^(number of zeros in the mask) - 2

Examples:

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#Example of dividing a network (subnetting)

We have the network:

192.168.1.0/24

We want 4 subnets. /24 → split into /26:

Subnets:

Each subnet contains 64 addresses, 62 hosts.

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#Subnet = broadcast domain

All devices in one subnet:

• are in the same broadcast domain
• can communicate directly (without a router)
• use ARP to find each other

To cross subnet boundaries, a router or VLAN routing is required.

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#Ready answer for an interview

A subnet is a logically distinct part of an IP network, defined by the subnet mask. It sets the address range, broadcast behavior, and the boundaries of the broadcast domain. All hosts within a subnet can communicate directly, while a router is required to connect different subnets. Subnets are used for network segmentation, security, and routing optimization.

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#Cheat sheet (5 seconds)

• Subnet = part of a network, defined by the mask.
• Contains: network, hosts, broadcast.
• All hosts → one broadcast domain.
• A router is needed to connect subnets.
• Used for segmentation, security, and optimization.