CIDR calculator

• Usable hosts is every address minus two: the first is the network address and the last is the broadcast address, so a /24 has 256 addresses and 254 usable hosts. Two prefixes break the rule. In a /31, both addresses are hosts on a point-to-point link (RFC 3021), so it has 2. A /32 is one host.
• Type says which range the block sits in. Use a private range (RFC 1918) for a VPC or office network: 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16. Watch the middle one: it ends at 172.31.255.255, so 172.32.0.0 is a public address someone else owns. "Shared address space" (100.64.0.0/10) belongs to carriers doing NAT, so keep it out of your own plans too.
• Address with host bits set. Type 10.0.0.5/24 and the calculator tells you it's in 10.0.0.0/24, then shows that network. A bare address with no prefix is read as a /32.

IP range to CIDR

Type a range as two addresses with a dash, like 10.0.0.5 - 10.0.1.10. A block always starts on a multiple of its own size, so most ranges don't fit one block. You get the fewest blocks that cover the range exactly: that one takes ten, from 10.0.0.5/32 to 10.0.1.10/32. That's the list a firewall rule or an allow list wants.

Take a VPC at 10.0.0.0/16 and split it into /24s: 256 subnets of 254 usable hosts. Use the first three for your web, app and database tiers, and leave the rest for whatever comes next. It's a dull plan, and that's the point: the third octet tells you which tier an address is in, and there's room for 253 more.

On American Cloud, you create the VPC with its CIDR, then add a tier per subnet. Each tier asks for a gateway and a netmask, and those are the Gateway and Netmask columns the calculator gives you. The gateway is the subnet's first usable address, so the web tier above gets 10.0.0.1 and 255.255.255.0.

Give every network you'll connect a block that doesn't overlap the others. Two networks both on 10.1.1.0/24 can't be joined without IP conflicts, and the fix is a new network.

Leave headroom in each subnet, too. Some clouds keep addresses for themselves. AWS, for example, holds back five in every subnet (the network address, the next three for its router, DNS and future use, and the last one) and allows subnets from /16 to /28, so a /28 there has 11 usable addresses, not 14.

Every prefix length from /32 to /0, with its netmask, wildcard mask and size. The usable hosts column uses the same rules as the calculator, /31 and /32 included.

An IPv4 address is 32 bits, and the prefix says how many of them, from the left, name the network (RFC 4632). The netmask is that many 1 bits, and the wildcard is the same mask flipped. The network address is your address with the host bits set to 0, and the broadcast address has them all set to 1. The block holds 2 to the power of (32 minus the prefix) addresses. The bar at the top of the result shows the split, bit by bit.

Everything runs in your browser. The math and the address types were checked on October 15, 2026 against RFC 4632, RFC 1918, RFC 3021, RFC 6598, RFC 5771, and the IANA special-purpose address registry. The AWS numbers come from AWS's subnet sizing docs. It handles IPv4 only.

A VPC on American Cloud is $12 a month, flat. If something doesn't connect the way you planned, our support team's median reply time is under 2 minutes. Need to check a domain while you're at it? Try the DNS lookup, or see pricing.