CIDR and Subnet Calculator

CIDR notation, like 192.168.1.0/24, describes a block of IP addresses. The number after the slash says how many leading bits belong to the network.

Enter a CIDR block, or an address with a subnet mask, to see the network and broadcast addresses, the mask, and the range of usable hosts.

Key features

• Network, broadcast, mask and wildcard mask
• First and last usable host
• Total and usable address counts
• Accepts CIDR or IP plus subnet mask

Example

192.168.1.77/24
Network   192.168.1.0
Broadcast 192.168.1.255
Hosts     192.168.1.1 - 192.168.1.254

How to use

1. Enter an address such as 10.0.0.5/24.
2. Read the network details.
3. Copy the values you need.

Use cases

• Planning subnets for a VPC or office network
• Writing firewall and security group rules
• Checking whether two addresses are in the same network

In depth

CIDR notation, introduced by RFC 4632, replaced the old class A, B and C system with variable length prefixes. An IPv4 address has 32 bits, and a prefix of /n reserves the first n bits for the network. The remaining 32 minus n bits identify hosts, giving two to the power of that number addresses. A /24 leaves 8 host bits, so 256 addresses, and a /26 leaves 6 bits, so 64. The subnet mask is simply n ones followed by zeros, and the wildcard mask, used by some routers and access lists, is its bitwise inverse.

To find the network address, apply a bitwise AND between the address and the mask. The broadcast address sets all host bits to one. For 10.1.5.130/26, the block size is 64, so subnets start at 0, 64, 128 and 192 in the last octet, which puts that address in 10.1.5.128 with broadcast 10.1.5.191. Usable hosts run from .129 to .190, which is 62 addresses. Special cases exist: RFC 3021 allows /31 point to point links to use both addresses, and /32 describes one host. Cloud providers often reserve additional addresses in each subnet, so check their rules.

Tips

• Subnet blocks must start on a multiple of their size. If a calculated network address does not match what you typed, your address was inside a block, not at its start.
• When planning a VPC, leave room to grow. Splitting 10.0.0.0/16 into /24 subnets gives 256 subnets of 254 hosts, but cloud providers typically reserve a few addresses per subnet.
• Double check mask versus wildcard when writing router or firewall rules. A mask of 255.255.255.0 corresponds to a wildcard of 0.0.0.255, and mixing them up changes which addresses match.

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