IP Subnet Calculator

Network engineers carving a VPC into subnets, sysadmins writing firewall rules, and developers debugging a Docker or Kubernetes network all need the same core facts: given this address and prefix, what is the network, what is the broadcast, which host range is usable, and does the address fall in a private or public block.

Paste an IPv4 address with an optional CIDR prefix and this calculator returns the full subnet breakdown, plus a mask-to-CIDR converter, a wildcard-to-mask inverter, and a range-to-CIDR expander. All arithmetic runs locally, so internal network ranges never leave the machine.

How the subnet math works

An IPv4 address is a 32-bit unsigned integer. The CIDR prefix tells you how many of those high bits belong to the network. The mask is built by setting the top prefix bits to one and the remaining host bits to zero, then the network address is the bitwise AND of the IP and the mask. The broadcast is the bitwise OR of the network and the wildcard, where the wildcard is the exact inverse of the mask.

Total hosts is two raised to the host-bit count. For prefixes of thirty or fewer, usable hosts subtracts two for the reserved network and broadcast addresses. A /31 is the point-to-point exception where both addresses are usable, and a /32 is a single host route. The calculator applies these rules automatically so the numbers match what your router or cloud console reports.

The binary view shows the same four values as dotted groups of eight bits, which is the fastest way to see which octet the prefix boundary falls in and why a /22 spans four third-octet addresses while a /23 spans only two.

Before and after: a typical /24 calculation

Input, a host address inside a private /24:

192.168.1.100/24

Output, the full breakdown:

network:   192.168.1.0/24
broadcast: 192.168.1.255
first:     192.168.1.1
last:      192.168.1.254
usable:    254
total:     256
mask:      255.255.255.0
wildcard:  0.0.0.255
class:     C
scope:     private (RFC 1918)

Who uses this subnet calculator

Common mistakes when calculating subnets

• Using the broadcast address as a host. The last address in a /24 is the broadcast and cannot be assigned to a device. Always use the last usable host shown in the calculator, which is one less than the broadcast for prefixes of /30 and shorter.
• Forgetting the /31 point-to-point exception. Older documentation says a /31 has no usable hosts. That was true before RFC 3021, but modern routers use /31 on point-to-point links and both addresses are assignable. The calculator reflects the modern behavior.
• Confusing a wildcard mask with a subnet mask. A wildcard is the bitwise inverse, so 0.0.0.255 is a wildcard, not a mask. If you paste a wildcard into a mask field you get a nonsensical result. Use the dedicated wildcard-to-mask box to invert it first.
• Assuming private addresses always start with 10. RFC 1918 defines three private blocks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. The 172.16 block is easy to miss because it only spans 172.16 through 172.31. The scope badge tells you which block an address falls in.

Best practices for subnet planning

• 1.Leave room to grow. Size each subnet one or two prefix lengths larger than your current need so you do not re-IP when headcount doubles.
• 2.Document the first and last usable host for every subnet in your runbook, so on-call engineers do not have to recompute them during an incident.
• 3.Use /31 for point-to-point links and /32 for loopback interfaces to conserve address space in large designs.
• 4.Expand DHCP ranges with the range-to-CIDR tool so your scopes align to clean prefix boundaries and routing summarization works.

Frequently asked questions