Step 12 — Quick Reference & Cheat Sheet
Congratulations on completing the Subnetting 101 masterclass! This page is your reference sheet. Bookmark it for quick access during network administration tasks.
Powers of 2 (Memorize This)
| 2⁰ | 2¹ | 2² | 2³ | 2⁴ | 2⁵ | 2⁶ | 2⁷ | 2⁸ |
|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 4 | 8 | 16 | 32 | 64 | 128 | 256 |
CIDR / Subnet Mask Reference
| CIDR | Subnet Mask | Addresses | Usable Hosts |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /17 | 255.255.128.0 | 32,768 | 32,766 |
| /18 | 255.255.192.0 | 16,384 | 16,382 |
| /19 | 255.255.224.0 | 8,192 | 8,190 |
| /20 | 255.255.240.0 | 4,096 | 4,094 |
| /21 | 255.255.248.0 | 2,048 | 2,046 |
| /22 | 255.255.252.0 | 1,024 | 1,022 |
| /23 | 255.255.254.0 | 512 | 510 |
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | 2* |
| /32 | 255.255.255.255 | 1 | 1 |
* /31 is point-to-point only (RFC 3021)
Block Size Quick Reference
| Mask Value | 0 | 128 | 192 | 224 | 240 | 248 | 252 |
|---|---|---|---|---|---|---|---|
| Block Size | 256 | 128 | 64 | 32 | 16 | 8 | 4 |
Formula: Block Size = 256 - Subnet Mask Value
Example: 255.255.255.192 → Block size = 256 - 192 = 64
Private IP Ranges (RFC 1918)
| Class | CIDR Block | Range | # Addresses |
|---|---|---|---|
| A | 10.0.0.0/8 | 10.0.0.0 - 10.255.255.255 | 16.7M |
| B | 172.16.0.0/12 | 172.16.0.0 - 172.31.255.255 | 1M |
| C | 192.168.0.0/16 | 192.168.0.0 - 192.168.255.255 | 65K |
Key Formulas
Usable Hosts
2host bits - 2
Number of Subnets
2borrowed bits
Host Bits
32 - CIDR prefix
Block Size
256 - mask value
Quick Subnetting Process
- Identify the "interesting" octet
/1-8 = 1st, /9-16 = 2nd, /17-24 = 3rd, /25-32 = 4th
- Calculate block size
256 - subnet mask value in interesting octet
- Find network address
Largest multiple of block size ≤ IP's octet value
- Find broadcast address
Network + block size - 1 (with .255 in remaining octets)
- Calculate host range
First host = Network + 1, Last host = Broadcast - 1
Common Mistakes to Avoid
- ✗Forgetting to subtract 2 for usable hosts
- ✗Starting subnets on wrong boundaries
- ✗Mixing up network and broadcast addresses
- ✗Not filling remaining octets with 0 (network) or 255 (broadcast)
- ✗VLSM: Not sorting by size (largest first)
🎉 Congratulations!
You've completed the Subnetting 101 Masterclass! You now have the skills to:
- Convert between binary, decimal, and CIDR notation
- Calculate network, broadcast, and host addresses
- Design efficient addressing schemes using VLSM
- Summarize routes for cleaner routing tables
- Apply these skills to real-world network design
Practice regularly to keep these skills sharp. Subnetting is like a muscle — use it or lose it!
What's Next?
- Practice with online subnetting calculators to verify your work
- Try subnetting challenges on networking certification prep sites
- Apply these skills to real network documentation at work
- Learn IPv6 subnetting (similar concepts, bigger addresses)