Learning Objectives

• Master /24 subnet calculations
• Understand block sizes and boundaries
• Practice finding network and broadcast addresses

The /24 Subnet

The /24 subnet is the most common in real networks. It has:

• 256 total addresses
• 254 usable hosts
• Subnet mask: 255.255.255.0
• All action in the fourth octet

Block Size and Boundaries

A /24 has a block size of 256 in the fourth octet. This means:

• Network boundaries: 192.168.0.0, 192.168.1.0, 192.168.2.0, etc.
• Each /24 is a complete octet
• The network address always ends in .0
• The broadcast address always ends in .255

Finding Network and Broadcast

Given the address 10.42.5.137/24:

1. The network address is 10.42.5.0 (fourth octet is 0)
2. The broadcast address is 10.42.5.255 (fourth octet is 255)
3. Usable range: 10.42.5.1 to 10.42.5.254

It's that simple with /24!

Dividing a /24

You can split a /24 into smaller subnets:

| Subnets | Prefix | Block Size | Addresses per Subnet | |---------|--------|-----------|---------------------| | 2 | /25 | 128 | 128 | | 4 | /26 | 64 | 64 | | 8 | /27 | 32 | 32 | | 16 | /28 | 16 | 16 | | 32 | /29 | 8 | 8 | | 64 | /30 | 4 | 4 |

Example: Split 192.168.1.0/24 into /28s

1st /28: 192.168.1.0 - 192.168.1.15 2nd /28: 192.168.1.16 - 192.168.1.31 3rd /28: 192.168.1.32 - 192.168.1.47 ... 16th /28: 192.168.1.240 - 192.168.1.255

Each /28 sub-subnet follows the block size of 16.

Real-World Example

A typical home network: 192.168.1.0/24

• Router: 192.168.1.1 (first usable host)
• DHCP range: 192.168.1.100 to 192.168.1.200
• Broadcast: 192.168.1.255
• Up to 254 devices

Try it yourself: enter 192.168.1.0/24 in the Subnet Calculator to see the full breakdown.

Key Takeaways

• /24 is the most common subnet size
• Fourth octet only — simple math
• Network = .0, Broadcast = .255
• Can be split into 2, 4, 8, 16, 32, or 64 smaller subnets

Next lesson: /25 through /30 subnets.