CIDR Reference Table
| Bits | Prefix | Subnet Mask | IP Addresses | Usable Hosts |
|---|---|---|---|---|
| 0 | /32 | 255.255.255.255 | 1 | N/A |
| 1 | /31 | 255.255.255.254 | 2 | 2 |
| 2 | /30 | 255.255.255.252 | 4 | 2 |
| 3 | /29 | 255.255.255.248 | 8 | 6 |
| 4 | /28 | 255.255.255.240 | 16 | 14 |
| 5 | /27 | 255.255.255.224 | 32 | 30 |
| 6 | /26 | 255.255.255.192 | 64 | 62 |
| 7 | /25 | 255.255.255.128 | 128 | 126 |
| 8 | /24 | 255.255.255.0 | 256 | 254 |
| 9 | /23 | 255.255.254.0 | 512 | 510 |
| 10 | /22 | 255.255.252.0 | 1 K | 1,022 |
| 11 | /21 | 255.255.248.0 | 2 K | 2,046 |
| 12 | /20 | 255.255.240.0 | 4 K | 4,094 |
| 13 | /19 | 255.255.224.0 | 8 K | 8,190 |
| 14 | /18 | 255.255.192.0 | 16 K | 16,382 |
| 15 | /17 | 255.255.128.0 | 32 K | 32,766 |
| 16 | /16 | 255.255.0.0 | 64 K | 65,534 |
| 17 | /15 | 255.254.0.0 | 128 K | 131,070 |
| 18 | /14 | 255.252.0.0 | 256 K | 262,142 |
| 19 | /13 | 255.248.0.0 | 512 K | 524,286 |
| 20 | /12 | 255.240.0.0 | 1 M | 1,048,574 |
| 21 | /11 | 255.224.0.0 | 2 M | 2,097,150 |
| 22 | /10 | 255.192.0.0 | 4 M | 4,194,302 |
| 23 | /9 | 255.128.0.0 | 8 M | 8,388,606 |
| 24 | /8 | 255.0.0.0 | 16 M | 16,777,214 |
| 25 | /7 | 254.0.0.0 | 32 M | 33,554,430 |
| 26 | /6 | 252.0.0.0 | 64 M | 67,108,862 |
| 27 | /5 | 248.0.0.0 | 128 M | 134,217,726 |
| 28 | /4 | 240.0.0.0 | 256 M | 268,435,454 |
| 29 | /3 | 224.0.0.0 | 512 M | 536,870,910 |
| 30 | /2 | 192.0.0.0 | 1,024 M | 1,073,741,822 |
| 31 | /1 | 128.0.0.0 | 2,048 M | 2,147,483,646 |
| 32 | /0 | 0.0.0.0 | 4,096 M | 4,294,967,294 |
K = 1,024 and M = 1,048,576 (binary multiples)
Common Subnet Sizes
/24 Network
256 IPs, 254 usableMost common for small LANs
/30 Network
4 IPs, 2 usablePoint-to-point links
/16 Network
65,536 IPs, 65,534 usableLarge enterprise or AWS VPC
Formula for Usable Hosts
Usable Hosts = 2n - 2 Where:- n = number of host bits
- The -2 accounts for the network and broadcast addresses
Example Calculation
For a /24 network:- Host bits = 32 - 24 = 8
- Total IPs = 28 = 256
- Usable hosts = 256 - 2 = 254
Practical Examples
- Small Office
- Point-to-Point Link
- AWS VPC
Network: 192.168.1.0/24
- Subnet Mask: 255.255.255.0
- Network Address: 192.168.1.0
- First Usable: 192.168.1.1
- Last Usable: 192.168.1.254
- Broadcast: 192.168.1.255
- Usable Hosts: 254
Quick Reference Guide
Most Common CIDR Blocks
Most Common CIDR Blocks
- /32 - Single host (1 IP)
- /31 - Point-to-point link (2 IPs, RFC 3021)
- /30 - Point-to-point link (2 usable IPs)
- /29 - Small subnet (6 usable IPs)
- /28 - Small subnet (14 usable IPs)
- /27 - Small subnet (30 usable IPs)
- /26 - Medium subnet (62 usable IPs)
- /25 - Medium subnet (126 usable IPs)
- /24 - Standard LAN (254 usable IPs)
- /23 - Medium LAN (510 usable IPs)
- /22 - Large LAN (1,022 usable IPs)
- /21 - Very large LAN (2,046 usable IPs)
- /20 - Small enterprise (4,094 usable IPs)
- /16 - Large enterprise/VPC (65,534 usable IPs)
- /8 - Class A network (16,777,214 usable IPs)
AWS VPC CIDR Recommendations
AWS VPC CIDR Recommendations
AWS allows VPC CIDR blocks between /16 and /28:
- /16 - Maximum size VPC (65,536 IPs)
- /20 - Medium VPC (4,096 IPs)
- /24 - Small VPC (256 IPs)
- /28 - Minimum size VPC (16 IPs)
Related Tools
VLSM Calculator
Variable-Length Subnet Masking calculator
VPC Basics
Learn about AWS VPC subnetting