---
id: 1777971486-WJIG
aliases:
  - Subnetting (Part 3)
tags: []
---

# Subnetting (Part 3 - VLSM)

## Class B Reference Table

| Prefix | Subnets | Hosts |
|--------|---------|-------|
| /17    | 2       | 32766 |
| /18    | 4       | 16382 |
| /19    | 8       | 8190  |
| /20    | 16      | 4094  |
| /21    | 32      | 2046  |
| /22    | 64      | 1022  |
| /23    | 128     | 510   |
| /24    | 256     | 254   |

---

## Class C Reference Table

| Prefix | Subnets | Hosts |
|--------|---------|-------|
| /25    | 2       | 126   |
| /26    | 4       | 62    |
| /27    | 8       | 30    |
| /28    | 16      | 14    |
| /29    | 32      | 6     |
| /30    | 64      | 2     |
| /31    |         | (0)   |
| /32    |         | (0–2) |

---

## Quiz Question 1

You have been given the network **172.30.0.0/16**. Your company requires  
**100 subnets with at least 500 hosts per subnet**.

**Answer:**
- Prefix: `/23`
- Network: `172.30.0.0/23`
- Subnet mask: `255.255.254.0`

---

## Quiz Question 2

What subnet does host **172.21.111.201/20** belong to?

```

10101100.00010101.01101111.11001001  (IP)
10101100.00010101.01100000.00000000  (Network)

```

**Answer:** `172.21.96.0/20`

---

## Quiz Question 3

What is the broadcast address of the network **192.168.91.78/26**?

```

11000000.10101000.10110011.01001110  (IP)
11000000.10101000.10110011.01111111  (Broadcast)

```

**Answer:** `192.168.91.127`

---

## Quiz Question 4

You divide the **172.16.0.0/16** network into 4 equal subnets.  
Identify the network and broadcast addresses.

### Subnet 1
- Network: `172.16.0.0/18`
- Broadcast: `172.16.63.255`

### Subnet 2
- Network: `172.16.64.0/18`
- Broadcast: `172.16.127.255`

### Subnet 3
- Network: `172.16.128.0/18`
- Broadcast: `172.16.191.255`

### Subnet 4
- Network: `172.16.192.0/18`
- Broadcast: `172.16.255.255`

---

## Quiz Question 5

You divide the **172.30.0.0/16** network into subnets of **1000 hosts each**.

**Answer:** 64 subnets

---

## Subnetting Class A Networks

The process of subnetting Class A, B, and C networks is exactly the same.

---

## Example 1

You are given the network **10.0.0.0/8**.  
You must create **2000 subnets**.

### Solution


**Answer:**
- Prefix: `/19`
- Usable hosts: `8192 - 2 = 8190`

---

## Example 2

PC1 has IP address **10.217.182.223/11**

### Subnet Details

- Network: `10.192.0.0/11`
- Broadcast: `10.223.255.255`
- First usable: `10.192.0.1`
- Last usable: `10.223.255.254`
- Usable hosts: `2,097,150`

---

## VLSM (Variable-Length Subnet Masks)

- Previously, we used **FLSM (Fixed-Length Subnet Masks)**  
- All subnets had the same prefix length  
- VLSM allows **different subnet sizes** for better efficiency  
- More flexible, slightly more complex  

---

## Example (VLSM Design)

```
Tokyo Lan A  = 110 Hosts  ---  Tokyo Lan B  = 8 Hosts
                           |
                         Router
                           |
                         Router
                           |
Toronto Lan A  = 29 Hosts --- Toronto Lan B  = 45 Hosts
```

We must divide **192.168.1.0/24** into 5 subnets:

- Tokyo LAN A → 110 hosts  
- Tokyo LAN B → 8 hosts  
- Toronto LAN A → 29 hosts  
- Toronto LAN B → 45 hosts  
- Point-to-point link  

### Steps

1. Assign largest subnet first  
2. Continue in descending order  
3. Repeat until done  

### Allocation Order

Tokyo A → Toronto B → Toronto A → Tokyo B → Point-to-point  

---

### Tokyo LAN A
- Network: `192.168.1.0/25`
- Broadcast: `192.168.1.127`
- Usable: `192.168.1.1 – 192.168.1.126`
- Hosts: 126  

---

### Toronto LAN B
- Network: `192.168.1.128/26`
- Broadcast: `192.168.1.191`
- Usable: `192.168.1.129 – 192.168.1.190`
- Hosts: 62  

---

### Toronto LAN A
- Network: `192.168.1.192/27`
- Broadcast: `192.168.1.223`
- Usable: `192.168.1.193 – 192.168.1.222`
- Hosts: 30  

---

### Tokyo LAN B
- Network: `192.168.1.224/28`
- Broadcast: `192.168.1.239`
- Usable: `192.168.1.225 – 192.168.1.238`
- Hosts: 14  

---

### Point-to-Point
- Network: `192.168.1.240/30`
- Broadcast: `192.168.1.243`
- Usable: `192.168.1.241 – 192.168.1.242`
- Hosts: 2  

---

## Additional Practice

- https://www.subnettingquestions.com  
- https://www.subnetting.org  
- https://www.subnettingpractice.com  
```