Subnet Calculator
Enter an IP address and CIDR prefix or subnet mask
Binary representation
Subnet mask cheat sheet
| CIDR | Mask | Hosts | Typical use |
|---|---|---|---|
| /32 | 255.255.255.255 | 1 | Single host |
| /30 | 255.255.255.252 | 2 | Point-to-point link |
| /28 | 255.255.255.240 | 14 | Small office |
| /24 | 255.255.255.0 | 254 | Standard LAN |
| /20 | 255.255.240.0 | 4094 | Large office / cloud VPC |
| /16 | 255.255.0.0 | 65534 | Class B network |
| /8 | 255.0.0.0 | 16M+ | Class A network |
CIDR notation
CIDR (Classless Inter-Domain Routing) replaced the old class-based system in 1993. Instead of being stuck with /8, /16, or /24 boundaries, you can allocate any prefix length from /0 to /32.
The number after the slash is how many bits are the network portion. 192.168.1.0/24 means the first 24 bits identify the network, leaving 8 bits for hosts. That gives you 28 - 2 = 254 usable addresses (minus network and broadcast).
For /31 and /32, the rules are different. /31 subnets (RFC 3021) have 2 addresses with no broadcast - used for point-to-point links. /32 is a single host route.
Private IP ranges (RFC 1918)
| Range | CIDR | Addresses | Common use |
|---|---|---|---|
| 10.0.0.0 - 10.255.255.255 | 10.0.0.0/8 | 16.7M | Cloud VPCs, large corporate networks |
| 172.16.0.0 - 172.31.255.255 | 172.16.0.0/12 | 1M | Docker default, medium networks |
| 192.168.0.0 - 192.168.255.255 | 192.168.0.0/16 | 65K | Home networks, small offices |
Also worth knowing: 169.254.0.0/16 is link-local (APIPA), 127.0.0.0/8 is loopback, and 100.64.0.0/10 is carrier-grade NAT (RFC 6598). These aren't routable on the public internet.
Choosing the right subnet size
Leave room to grow. If you have 20 hosts, don't use a /27 (30 usable). Use a /26 (62 usable) or /25 (126). Renumbering a network is painful.
Cloud providers reserve addresses. AWS reserves 5 IPs per subnet (network, router, DNS, future, broadcast). A /28 that should have 14 hosts only gives you 11 on AWS.
Separate concerns with subnets. Put web servers, databases, and management on different subnets. It's easier to apply security groups and ACLs at the subnet level than per host.
Document everything. IP allocation gets messy fast. Track which subnets are assigned where, or you'll end up with overlapping ranges when you peer VPCs or set up VPN tunnels.
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