CIDR Subnet Calculator
Calculate subnet ranges, usable IPs, broadcast addresses, network masks, and binary representations from any CIDR notation.
Why CIDR math matters for infrastructure
CIDR (Classless Inter-Domain Routing) notation like 10.0.0.0/16 describes a range of IP addresses using a network prefix and a subnet mask. Getting the math right is essential when you carve a VPC into subnets, size a Kubernetes pod network, write firewall rules or define network policies. A prefix that is too small starves you of addresses; one that is too large wastes space and can overlap with peered networks, which breaks routing in ways that are painful to debug later.
Read the mask, hosts and ranges at a glance
This calculator turns a CIDR block into its netmask, network address, broadcast address, usable host range and total host count. That lets you confirm, before you apply Terraform or a cloud console change, that a subnet has enough room for autoscaling nodes and that two ranges do not collide. Everything is computed locally in your browser, so you can plan private address space without sending any of it to a server.
Common CIDR mistakes and how to fix them
The classic error is overlapping ranges between a VPC and a VPN or peered network, which silently blackholes traffic. Another is forgetting that the network and broadcast addresses are not usable hosts, so a /30 gives you two usable IPs, not four. When planning cluster networking, keep the pod CIDR, service CIDR and node subnet distinct and non-overlapping. For tuning the proxy layer that sits in front of those networks, see our Nginx reverse proxy performance tuning guide.
Frequently asked questions
How many hosts are in a /24?
A /24 has 256 addresses and 254 usable hosts, because the network and broadcast addresses are reserved.
What does the slash number mean?
It is the number of bits fixed as the network prefix. A larger number means a smaller network with fewer hosts.
Why must subnets not overlap?
Overlapping ranges make routing ambiguous, so traffic can be dropped or sent to the wrong network.
Is a /16 big enough for a Kubernetes cluster?
Usually yes for the pod network, but confirm your per-node pod count and node total against the address space.