Breakdown

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What is a Subnet?

A subnet (short for sub-network) is a smaller, segmented portion of a larger network. It helps organize and manage IP addresses efficiently, reduces network congestion, and improves security.

IP Addresses and Subnet Masks

Every device on a network has an IP address, which is like a home address. An IP address consists of two parts:

  1. Network portion – Identifies which network or subnet the device belongs to.
  2. Host portion – Identifies the specific device within that network.

A subnet mask is what determines how much of the IP address is for the network and how much is for the host.

Example:

  • IP Address: 192.168.1.10
  • Subnet Mask: 255.255.255.0
    • The 255.255.255.0 means that the first three octets (192.168.1) are the network portion.
    • The last octet (.10) is the host portion.

This means all devices with 192.168.1.X belong to the same subnet.

Subnet Mask Breakdown

Subnet masks define how many bits are used for the network and how many for hosts.

Subnet Mask
CIDR Notation
Max Hosts per Subnet
255.0.0.0
/8
16,777,214
255.255.0.0
/16
65,534
255.255.255.0
/24
254
255.255.255.128
/25
126
255.255.255.192
/26
62

💡 CIDR notation (like /24) simply tells us how many bits are reserved for the network.

  • /24 (255.255.255.0) → 256 total addresses, 254 usable (first = network, last = broadcast).
  • /26 (255.255.255.192) → 64 total addresses, 62 usable.

Why Subnet?

  1. Efficient Use of IP Addresses
    • Instead of wasting thousands of IPs on a small office, subnetting allows for better allocation.
  2. Improves Network Performance
    • Less congestion because traffic stays within a smaller area.
  3. Enhances Security
    • You can isolate critical devices (e.g., servers in one subnet, user devices in another).
  4. Easier Network Management
    • Different departments (HR, IT, Finance) can have separate subnets for better control.

Subnet Example in Action

Let’s say a company has 192.168.1.0/24 and needs to split it into two subnets:

  1. Subnet 1:
    • Network: 192.168.1.0/25
    • Range: 192.168.1.1 - 192.168.1.126
    • Broadcast: 192.168.1.127
  2. Subnet 2:
    • Network: 192.168.1.128/25
    • Range: 192.168.1.129 - 192.168.1.254
    • Broadcast: 192.168.1.255

Each subnet has 126 usable hosts.