What Is CIDR Notation?
CIDR (Classless Inter-Domain Routing) notation is the standard way to express IP address ranges. It uses a slash followed by a number (e.g., /24) to indicate how many bits are used for the network portion, which determines the size of the address block.
Complete CIDR Reference Table
| CIDR | Subnet Mask | Total IPs | Usable IPs | Common Use Case |
|---|---|---|---|---|
| /32 | 255.255.255.255 | 1 | 1 | Single host |
| /31 | 255.255.255.254 | 2 | 2 | Point-to-point link |
| /30 | 255.255.255.252 | 4 | 2 | Small WAN link |
| /29 | 255.255.255.248 | 8 | 6 | Small office |
| /28 | 255.255.255.240 | 16 | 14 | Small network segment |
| /27 | 255.255.255.224 | 32 | 30 | Department network |
| /26 | 255.255.255.192 | 64 | 62 | Small datacenter |
| /25 | 255.255.255.128 | 128 | 126 | Medium network |
| /24 | 255.255.255.0 | 256 | 254 | Standard network, minimum routable |
| /23 | 255.255.254.0 | 512 | 510 | Small-medium ISP |
| /22 | 255.255.252.0 | 1,024 | 1,022 | Medium infrastructure |
| /21 | 255.255.248.0 | 2,048 | 2,046 | Datacenter, hosting |
| /20 | 255.255.240.0 | 4,096 | 4,094 | Large hosting, ISP |
| /19 | 255.255.224.0 | 8,192 | 8,190 | Regional ISP |
| /18 | 255.255.192.0 | 16,384 | 16,382 | Large ISP |
| /17 | 255.255.128.0 | 32,768 | 32,766 | Major provider |
| /16 | 255.255.0.0 | 65,536 | 65,534 | Enterprise, large ISP |
Key Concepts
Network and Broadcast Addresses
In most subnets (/30 and larger), two addresses are reserved: the first address is the network address, and the last is the broadcast address. This is why a /24 has 256 total IPs but only 254 usable.
The /24 Minimum
A /24 (256 IPs) is generally the smallest block that can be independently routed on the public internet. Most ISPs will not accept BGP announcements for prefixes smaller than /24. This makes /24 the de facto minimum purchase size for IPv4 blocks.
Quick Calculation Formula
To calculate the number of IPs: 2^(32 - CIDR) = Total IPs
- /24 = 2^(32-24) = 2^8 = 256 IPs
- /22 = 2^(32-22) = 2^10 = 1,024 IPs
- /20 = 2^(32-20) = 2^12 = 4,096 IPs
- /16 = 2^(32-16) = 2^16 = 65,536 IPs
Choosing the Right Subnet Size
Consider these factors:
- Current needs + 20% buffer: Plan for some growth
- Budget: Larger blocks cost more total but less per IP
- Routability: /24 is the minimum for public routing
- Aggregation: Buying contiguous blocks allows aggregation for cleaner routing
Browse our marketplace to find blocks in the size you need, or start your purchase today.