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Subnetting (Part 2)
Problem
You have a network topology where each segment requires 45 hosts:
45 hosts -> SW1 -> R1 <- SW3 <- 45 hosts
45 hosts -> SW2 -> R1 <- SW4 <- 45 hosts
You are given the network:
- 192.168.1.0/24
Your task is to divide it into 4 subnets, each capable of supporting at least 45 hosts.
Step 1: Choose the Right Subnet Size
To support 45 hosts, we need:
- ( 2^n - 2 \geq 45 )
- ( n = 6 ) → ( 2^6 - 2 = 62 ) hosts
So each subnet must be a /26.
Step 2: Subnet Breakdown (/26)
Each subnet increases by 64 addresses.
Subnet 1
- Network: 192.168.1.0/26
- Range: 192.168.1.0 – 192.168.1.63
- Broadcast: 192.168.1.63
Binary:
11000000.10101000.00000001.00 000000
Subnet 2
- Network: 192.168.1.64/26
- Range: 192.168.1.64 – 192.168.1.127
- Broadcast: 192.168.1.127
Subnet 3
- Network: 192.168.1.128/26
- Range: 192.168.1.128 – 192.168.1.191
- Broadcast: 192.168.1.191
Subnet 4
- Network: 192.168.1.192/26
- Range: 192.168.1.192 – 192.168.1.255
- Broadcast: 192.168.1.255
Quick Trick 💡
Each subnet jumps by 64:
- 0 → 64 → 128 → 192
Think of it like stepping stones across a river. Same stride, different landing spots.
Subnetting Basics
-
Number of subnets: ( 2^x ) (x = borrowed bits)
-
Number of hosts per subnet: ( 2^n - 2 ) (n = host bits)
Identify the Subnet
Example 1
Host: 192.168.5.57/27
- /27 → block size = 32
- Subnets: 0, 32, 64, ...
57 falls between 32 and 63
→ Network: 192.168.5.32/27
Example 2
Host: 192.168.29.219/29
- /29 → block size = 8
- Subnets: 0, 8, 16, ..., 216, 224
219 falls between 216 and 223
→ Network: 192.168.29.216/29
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 |
Subnetting Class B Networks
The method is exactly the same. Only the starting mask changes.
Example 1
Network: 172.16.0.0/16 Required subnets: 80
- ( 2^7 = 128 ) → enough
- New prefix: /23
Example 2
Network: 172.22.0.0/16 Required subnets: 500
- ( 2^9 = 512 )
- New prefix: /25
Example 3
Network: 172.18.0.0/16 Required subnets: 250
- ( 2^8 = 256 )
- New prefix: /24
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 |
Final Review
- Subnetting is about borrowing bits
- Larger prefix = more subnets, fewer hosts
- Smaller prefix = fewer subnets, more hosts