CIDR (Classless Inter-Domain Routing) notation is a compact way to specify IP addresses and their associated network masks. Written as an IP address followed by a slash and a prefix length (e.g., 192.168.1.0/24), the prefix length indicates how many leading bits define the network portion, with the remaining bits available for host addresses.
Complete CIDR Reference Table
| CIDR | Subnet Mask | Total Addresses | Usable Hosts | Example Use |
|---|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 | Class A network (10.0.0.0/8) |
| /9 | 255.128.0.0 | 8,388,608 | 8,388,606 | Large ISP allocation |
| /10 | 255.192.0.0 | 4,194,304 | 4,194,302 | Large ISP allocation |
| /11 | 255.224.0.0 | 2,097,152 | 2,097,150 | Large ISP allocation |
| /12 | 255.240.0.0 | 1,048,576 | 1,048,574 | Private range (172.16.0.0/12) |
| /13 | 255.248.0.0 | 524,288 | 524,286 | Regional allocation |
| /14 | 255.252.0.0 | 262,144 | 262,142 | Regional allocation |
| /15 | 255.254.0.0 | 131,072 | 131,070 | Regional allocation |
| /16 | 255.255.0.0 | 65,536 | 65,534 | Class B network, AWS VPC default |
| /17 | 255.255.128.0 | 32,768 | 32,766 | Large campus network |
| /18 | 255.255.192.0 | 16,384 | 16,382 | Large campus network |
| /19 | 255.255.224.0 | 8,192 | 8,190 | ISP customer block |
| /20 | 255.255.240.0 | 4,096 | 4,094 | Large office or data center |
| /21 | 255.255.248.0 | 2,048 | 2,046 | Medium data center |
| /22 | 255.255.252.0 | 1,024 | 1,022 | Medium office network |
| /23 | 255.255.254.0 | 512 | 510 | Small data center |
| /24 | 255.255.255.0 | 256 | 254 | Standard LAN subnet |
| /25 | 255.255.255.128 | 128 | 126 | Half of a /24 subnet |
| /26 | 255.255.255.192 | 64 | 62 | Small office network |
| /27 | 255.255.255.224 | 32 | 30 | Small department or VLAN |
| /28 | 255.255.255.240 | 16 | 14 | Small subnet or DMZ |
| /29 | 255.255.255.248 | 8 | 6 | Small point-of-presence |
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point link |
| /31 | 255.255.255.254 | 2 | 2 | Point-to-point link (RFC 3021) |
| /32 | 255.255.255.255 | 1 | 1 | Single host route |
Common Subnet Sizes
1 address. Used for host routes, loopback addresses, and firewall rules targeting a specific IP.
2 addresses, both usable (RFC 3021). Designed for point-to-point links between two routers where no broadcast address is needed.
4 addresses, 2 usable. The traditional choice for point-to-point router links before /31 became widely supported.
256 addresses, 254 usable. The most common subnet size for local area networks, office floors, and VLANs.
65,536 addresses. Commonly used for entire VPCs, campus networks, or Class B allocations.
16,777,216 addresses. Reserved for the largest private networks (10.0.0.0/8) and legacy Class A assignments.
Private IP Ranges (RFC 1918)
These address ranges are reserved for private networks and are not routable on the public internet. Any organization can use them internally without coordination.
| CIDR Block | Address Range | Total Addresses | Class | Typical Use |
|---|---|---|---|---|
| 10.0.0.0/8 | 10.0.0.0 -- 10.255.255.255 | 16,777,216 | Class A | Large enterprises, cloud VPCs |
| 172.16.0.0/12 | 172.16.0.0 -- 172.31.255.255 | 1,048,576 | Class B | Medium networks, Docker default |
| 192.168.0.0/16 | 192.168.0.0 -- 192.168.255.255 | 65,536 | Class C | Home and small office networks |
Cloud Provider Defaults
| Provider | Resource | Default CIDR | Notes |
|---|---|---|---|
| AWS | VPC | /16 (e.g., 10.0.0.0/16) | Supports /16 to /28; /16 is the default and recommended size |
| AWS | Subnet | /24 (e.g., 10.0.1.0/24) | AWS reserves 5 IPs per subnet (first 4 + last 1) |
| GCP | VPC Subnet | Varies by region | Auto-mode VPCs use /20 subnets; custom mode allows /29 to /8 |
| Azure | VNet | /16 (e.g., 10.0.0.0/16) | Supports /8 to /29; Azure reserves 5 IPs per subnet |
Quick Mental Math
Each decrease in prefix length by 1 doubles the number of addresses. A /23 has twice as many addresses as a /24.
Total addresses = 2^(32 - prefix). For /24: 2^(32-24) = 2^8 = 256 addresses.
For /31 and /32, all addresses are usable. For all other prefixes, subtract 2 from the total (network address + broadcast address).
To split a /24 into two subnets, use two /25s. To split into four, use four /26s. Each additional bit doubles the number of subnets.
Calculate subnets, host ranges, and CIDR breakdowns with the IP Subnet Calculator tool.