CIDR & Subnet Calculator (with AWS VPC Mode)
Enter any IPv4 CIDR block to get its network, broadcast, netmask and usable host range. AWS VPC mode accounts for the five addresses AWS reserves in every subnet, and you can split a VPC into subnets and check two ranges for overlap.
Free, no signup. Runs entirely in your browser; nothing you enter is sent anywhere.
Split into subnets
Each subnet is a /19 with 8,187 usable hosts.
- 10.0.0.0/19
- 10.0.32.0/19
- 10.0.64.0/19
- 10.0.96.0/19
- 10.0.128.0/19
- 10.0.160.0/19
Overlap check
These ranges overlap. Peering or VPN routing between them will not work.
Reading CIDR notation
In 10.0.0.0/16, the /16 says the first 16 bits are the network and the remaining 16 are for hosts, giving 216 = 65,536 addresses. Each step up halves the block: a /24 has 256 addresses, a /28 has 16. Classic networks lose two addresses per subnet to the network and broadcast addresses.
Planning a VPC that won't need re-addressing
AWS reserves five addresses in every subnet (the first four and the last), so a /28 leaves only 11 usable IPs. Size subnets for growth, especially for Kubernetes, where every Pod can take a VPC IP. Pick a VPC range that doesn't overlap your other VPCs, on-prem networks or VPN ranges: overlapping ranges can't be peered or routed later without a painful migration. Start with AWS VPC explained if this is your first VPC.
Questions people ask
How many usable IP addresses are in a /24?
254 on a classic network: 256 addresses minus the network and broadcast addresses. In an AWS VPC subnet it's 251, because AWS reserves five addresses in every subnet.
Why does AWS reserve 5 IP addresses per subnet?
AWS keeps the network address, the VPC router (base + 1), the DNS server (base + 2), one address reserved for future use (base + 3) and the last address in every subnet. A /24 subnet therefore has 251 usable addresses.
What CIDR block should I use for a VPC?
A /16 from private space such as 10.x.0.0/16 gives room for many subnets across availability zones. Choose a range that doesn't overlap other VPCs, on-prem networks or VPNs you may need to connect to, because overlapping ranges can't be routed together.