# Network Subnetting

#### Cheat Sheet

<table border="1" id="bkmrk-group-size-128-64-32" style="border-collapse: collapse; width: 100%;"><colgroup><col style="width: 11.7989%;"></col><col style="width: 10.3703%;"></col><col style="width: 11.0846%;"></col><col style="width: 11.0846%;"></col><col style="width: 11.0846%;"></col><col style="width: 11.0846%;"></col><col style="width: 11.0846%;"></col><col style="width: 11.0846%;"></col><col style="width: 11.0846%;"></col></colgroup><tbody><tr><td>Group Size</td><td>**128**</td><td>**64**</td><td>**32**</td><td>**16**</td><td>**8**</td><td>**4**</td><td>**2**</td><td>**1**</td></tr><tr><td>Subnet</td><td>**128**</td><td>**192**</td><td>**224**</td><td>**240**</td><td>**248**</td><td>**252**</td><td>**254**</td><td>**255**</td></tr><tr><td>CIDR (4o)</td><td>/25</td><td>/26</td><td>/27</td><td>/28</td><td>/29</td><td>/30</td><td>/31</td><td>/32</td></tr><tr><td>3rd octet</td><td>/17</td><td>/18</td><td>/19</td><td>/20</td><td>/21</td><td>/22</td><td>/23</td><td>/24</td></tr><tr><td>2nd octet</td><td>/9</td><td>/10</td><td>/11</td><td>/12</td><td>/13</td><td>/14</td><td>/15</td><td>/16</td></tr><tr><td>1st octet</td><td>/1</td><td>/2</td><td>/3</td><td>/4</td><td>/5</td><td>/6</td><td>/7</td><td>/8</td></tr></tbody></table>

1. Use CIDR/Mask to find group size above
2. Starting from 0, or the corresponding subnet value above, or one to its left, increase or decrease by the group size, until you pass the corresponding octet value in the target IP 
    1. Next Network ID cannot contain 256, instead increment the preceding octet by 1 and reset to 0
    2. If this is the 1st octet, Next Network is N/A, and Broadcast IP = 255.255.255.255
3. Number before target IP = Network ID
4. First Host = Network ID + 1
5. Number after target IP = Next Network ID
6. Broadcast = Next Network - 1
7. Last Host = Next Network - 2
8. \#IP Addresses = Group size, or 2<sup>(32-CIDR) </sup>for 3rd/4th octet

<table border="1" id="bkmrk-network-id-first-hos" style="border-collapse: collapse; width: 37.381%;"><colgroup><col style="width: 50%;"></col><col style="width: 50%;"></col></colgroup><tbody><tr><td>Network ID</td><td>  
</td></tr><tr><td>First Host IP</td><td>  
</td></tr><tr><td>Last Host IP</td><td>  
</td></tr><tr><td>Broadcast IP</td><td>  
</td></tr><tr><td>Next Network ID</td><td>  
</td></tr><tr><td>No. IP Addresses</td><td>  
</td></tr><tr><td>CIDR/Subnet Mask</td><td>  
</td></tr></tbody></table>

- East subnet bit doubles the available subnets
- Each host bit doubles the available host addresses

#### FLSM Questions

<table border="1" id="bkmrk-21-2-22-4-23-8-24-16" style="border-collapse: collapse; width: 10.119%; height: 308.417px;"><colgroup><col style="width: 39.3939%;"></col><col style="width: 60.6061%;"></col></colgroup><tbody><tr style="height: 31.8833px;"><td style="height: 31.8833px;">2<sup>1</sup></td><td style="height: 31.8833px;">2</td></tr><tr style="height: 31.8833px;"><td style="height: 31.8833px;">2<sup>2</sup></td><td style="height: 31.8833px;">4</td></tr><tr style="height: 31.8833px;"><td style="height: 31.8833px;">2<sup>3</sup></td><td style="height: 31.8833px;">8</td></tr><tr style="height: 31.8833px;"><td style="height: 31.8833px;">2<sup>4</sup></td><td style="height: 31.8833px;">16</td></tr><tr style="height: 31.8833px;"><td style="height: 31.8833px;">2<sup>5</sup></td><td style="height: 31.8833px;">32</td></tr><tr style="height: 29.8px;"><td style="height: 29.8px;">2<sup>6</sup></td><td style="height: 29.8px;">64</td></tr><tr style="height: 29.8px;"><td style="height: 29.8px;">2<sup>7</sup></td><td style="height: 29.8px;">128</td></tr><tr style="height: 29.8px;"><td style="height: 29.8px;">2<sup>8</sup></td><td style="height: 29.8px;">256</td></tr><tr style="height: 29.8px;"><td style="height: 29.8px;">2<sup>9</sup></td><td style="height: 29.8px;">512</td></tr></tbody></table>

Starting with /18, what size sub-network would you need to create 100 sub-networks? <span style="background-color: rgb(255, 255, 255); color: rgb(224, 62, 45);">/25</span>

- <span style="background-color: rgb(255, 255, 255); color: rgb(0, 0, 0);">Or, 128 = 2<span style="color: rgb(224, 62, 45);"><sup>7 </sup>/18 + 7 = /25</span></span>

[![image.png](https://note.human-bean.net/uploads/images/gallery/2026-07/scaled-1680-/2dGimage.png)](https://note.human-bean.net/uploads/images/gallery/2026-07/2dGimage.png)

Starting with /21, how many sub-networks could you create that could contain 50 IP addresses? <span style="color: rgb(224, 62, 45);">32</span>

- <span style="color: rgb(0, 0, 0);">Or, <span style="color: rgb(224, 62, 45);">/26 - /20</span> = 2<span style="color: rgb(224, 62, 45);"><sup>5</sup></span> = 32</span>

[![image.png](https://note.human-bean.net/uploads/images/gallery/2026-07/scaled-1680-/xQqimage.png)](https://note.human-bean.net/uploads/images/gallery/2026-07/xQqimage.png)

[![image.png](https://note.human-bean.net/uploads/images/gallery/2026-07/scaled-1680-/e9ximage.png)](https://note.human-bean.net/uploads/images/gallery/2026-07/e9ximage.png)

#### How to Read a Subnetting Question

You will generally be given 2, and your job will be to solve the 3rd.

- A starting network size
- Number of Sub-Networks
- Size of Sun-Network

#### Variable Length Subnet Masks (VLSM)

[https://www.youtube.com/watch?v=amKyfbg5G2Q](https://www.youtube.com/watch?v=amKyfbg5G2Q)