Subnetting 101

Step 6 — Calculating Host Ranges

Now that you can find network and broadcast addresses, calculating the usable host range is simple: everything in between.

The Simple Formula

First usable host:Network address + 1Last usable host:Broadcast address - 1

Example: 192.168.1.0/24

Network address: 192.168.1.0

First usable host: 192.168.1.1

Last usable host: 192.168.1.254

Broadcast address: 192.168.1.255

Complete Subnet Information

When analyzing a subnet, you should be able to determine all of this:

Example: 10.50.100.75/26

Given IP10.50.100.75
CIDR/26
Subnet Mask255.255.255.192
Network Address10.50.100.64
First Host10.50.100.65
Last Host10.50.100.126
Broadcast Address10.50.100.127
Usable Hosts62 (2⁶ - 2)

Worked Examples

172.16.45.200/21

Block size: 8 (in 3rd octet)

Network: 172.16.40.0 (45 falls in 40-47 block)

Broadcast: 172.16.47.255

Host range: 172.16.40.1 to 172.16.47.254

Usable hosts: 2¹¹ - 2 = 2,046

192.168.50.130/27

Block size: 32

Network: 192.168.50.128 (130 falls in 128-159 block)

Broadcast: 192.168.50.159

Host range: 192.168.50.129 to 192.168.50.158

Usable hosts: 2⁵ - 2 = 30

10.0.0.1/29

Block size: 8

Network: 10.0.0.0 (1 falls in 0-7 block)

Broadcast: 10.0.0.7

Host range: 10.0.0.1 to 10.0.0.6

Usable hosts: 2³ - 2 = 6

Quick Reference: Subnetting a /24

Here's what you get when you subnet a typical /24 network:

CIDRSubnetsHosts/SubnetExample Ranges
/241254.1-.254
/252126.1-.126, .129-.254
/26462.1-.62, .65-.126, .129-.190, .193-.254
/27830.1-.30, .33-.62, .65-.94, ...
/281614.1-.14, .17-.30, .33-.46, ...
/29326.1-.6, .9-.14, .17-.22, ...
/30642.1-.2, .5-.6, .9-.10, ...

Are Two IPs on the Same Subnet?

A common real-world task: determine if two hosts can communicate directly without a router.

Example: Can 192.168.1.100/26 communicate with 192.168.1.150/26?

Block size: 64

100 is in block 64-127 (network: 192.168.1.64)

150 is in block 128-191 (network: 192.168.1.128)

Different subnets! They need a router.

Example: Can 10.20.30.40/28 communicate with 10.20.30.35/28?

Block size: 16

40 is in block 32-47 (network: 10.20.30.32)

35 is in block 32-47 (network: 10.20.30.32)

Same subnet! Direct communication OK.

Practice Exercises

For each IP/CIDR, find the complete subnet info:

Store your work

These have multiple outputs (mask, network, host range, broadcast, usable hosts). Use this as your saved answer sheet.

Saved locally in your browser for this device.

  1. 192.168.100.200/28
  2. 10.10.10.10/30
  3. 172.16.50.100/23
Show Answers

1. 192.168.100.200/28

  • Mask: 255.255.255.240
  • Network: 192.168.100.192
  • Host range: 192.168.100.193 - 192.168.100.206
  • Broadcast: 192.168.100.207
  • Usable hosts: 14

2. 10.10.10.10/30

  • Mask: 255.255.255.252
  • Network: 10.10.10.8
  • Host range: 10.10.10.9 - 10.10.10.10
  • Broadcast: 10.10.10.11
  • Usable hosts: 2

3. 172.16.50.100/23

  • Mask: 255.255.254.0
  • Network: 172.16.50.0 (block size 2, 50 is in 50-51)
  • Host range: 172.16.50.1 - 172.16.51.254
  • Broadcast: 172.16.51.255
  • Usable hosts: 510

Checkpoint

Before moving on, make sure you can:

  • Calculate the complete subnet info for any IP/CIDR
  • Find first and last usable host addresses
  • Determine if two IPs are on the same subnet
  • Understand why network and broadcast can't be used for hosts