CIDR / subnet calculator
Enter a block like 10.0.0.0/16 and see its range and masks, split it into subnets, check overlaps or turn a range into CIDR blocks.
CIDR notation and subnet maths
Classless Inter-Domain Routing writes a network as an address and a prefix length: 10.0.0.0/16. Every cloud network, firewall rule, Kubernetes pod range and VPN route is expressed that way, and getting the arithmetic wrong means overlapping ranges, exhausted subnets or a security group that opens far more than intended.
The calculator covers the questions that come up most often. Calculate shows the network and broadcast address, first and last usable host, netmask, wildcard mask, address counts and the binary mask, and flags special ranges (private, loopback, carrier-grade NAT, documentation). Split divides a block into equal smaller subnets, which is how you carve a VPC into per-zone, per-tier subnets. Check tells you whether an IP address is inside a block and whether two blocks overlap, useful before peering networks. Range turns an arbitrary start and end address into the smallest list of CIDR blocks, and merges a list of blocks into the fewest equivalent ones.
Practical rules
- Pick VPC and cluster ranges that do not overlap with anything you may ever connect: other VPCs, on-premises networks, VPN clients.
- Leave room: a /24 per subnet is easy to reason about, and growing later means renumbering.
- Kubernetes needs separate ranges for nodes, pods and services; pod ranges are often large (/14 to /16).
- Never assume cloud providers give you every address; subtract the reserved ones.
Addresses are handled with arbitrary-precision integers, so IPv6 works the same way as IPv4. Everything runs in your browser.
Frequently asked questions
What does /24 mean?
The number after the slash is how many leading bits of the address identify the network. A /24 fixes the first 24 bits (the first three octets), leaving 8 bits for hosts: 256 addresses, of which 254 are usable because the first is the network address and the last is the broadcast.
Why are only 254 hosts usable in a /24?
The all-zeros host address names the network itself and the all-ones address is the broadcast address, so neither can be given to a host. /31 (RFC 3021, point-to-point links) and /32 (a single host) are special cases.
How many addresses does a cloud subnet really have?
Providers reserve a few addresses in every subnet. AWS reserves five (network, router, DNS, future use and broadcast), so a /24 gives 251; Azure reserves five as well and Google Cloud reserves four. Plan with these numbers.
What is the difference between IPv4 and IPv6 sizing?
IPv6 addresses are 128 bits. The standard subnet is a /64 (2^64 addresses) and there is no broadcast address, so every address in the block is usable.