CIDR notation compresses an IP address and its subnet mask into one compact form, like 10.0.0.1/24, but reading exactly what that slash number means for your network still requires converting it into a mask, a range, and a host count. This CIDR calculator does that conversion instantly: enter an IP address and mask bits (or pick a CIDR netmask directly), and it returns the wildcard mask, network address, broadcast range, and maximum subnets and addresses for that prefix length.

How to use this CIDR calculator

1. Enter an IP Address in the input field, for example 10.0.0.1.
2. Select the Mask Bits (the CIDR prefix length, such as /24) from the dropdown, or select the equivalent value directly from the CIDR Netmask dropdown, since the two are linked.
3. Review the Wildcard Mask, automatically calculated as the inverse of the subnet mask.
4. Check Maximum Subnets and Maximum Addresses to see how many subnets and host addresses that prefix length allows.
5. Read the CIDR Network (Route), the network address for that IP and prefix combination.
6. Check the Net: CIDR Notation field for the network address written in slash notation.
7. Review the CIDR Address Range, showing the full span of addresses from network to broadcast address for that subnet.

What this CIDR calculator does

Classless Inter-Domain Routing (CIDR) notation replaced the older, rigid Class A/B/C addressing system by allowing a subnet mask to be any prefix length rather than a fixed set of options. This calculator takes an IP address and a CIDR prefix (mask bits) and derives every other value that prefix length implies: the equivalent dotted-decimal subnet mask, its inverse (the wildcard mask), the network address for your specific IP, the full usable address range, and how many total subnets and addresses that prefix length supports.

Understanding CIDR notation and prefix length

A larger prefix number (like /28) means more bits are dedicated to identifying the network, leaving fewer bits for host addresses, so /28 describes a smaller subnet than /24. This inverse relationship, larger prefix number equals smaller subnet, is one of the more common points of confusion when reading CIDR notation for the first time, and it’s exactly why this calculator’s Maximum Addresses column is worth checking directly rather than trying to recall the relationship from memory.

How the wildcard mask is derived

The wildcard mask is the bitwise inverse of the subnet mask, flipping every 1-bit to a 0-bit and vice versa. It’s used in access control lists and OSPF configuration on many router platforms, where the matching logic is expressed in terms of “don’t care” bits (the wildcard’s 1-bits) rather than the network-identifying bits a subnet mask represents.

Maximum subnets and maximum addresses

Every subnet reserves one address for the network address itself and one for the broadcast address, which is why usable host counts run slightly below the raw total the remaining bits would otherwise suggest. This calculator’s Maximum Addresses field accounts for the full address block, while the CIDR Address Range shown separately marks out the network and broadcast boundaries so you can see exactly where the usable range sits.

Network address versus your entered IP

The CIDR Network (Route) and Net: CIDR Notation fields don’t just echo back your entered IP, they calculate the actual network address that IP belongs to under the selected prefix length. This distinction matters in real network configuration: entering 10.0.0.1/24 has you configuring a host address on the 10.0.0.0/24 network, and this calculator makes that underlying network boundary explicit rather than leaving you to work it out separately.

CIDR and VLSM: how they relate

VLSM (Variable Length Subnet Masking) is the broader practice of subnetting a larger network into multiple smaller subnets of different sizes, each sized to match its actual number of hosts, rather than carving every subnet to the same fixed size. CIDR notation is the addressing format that makes VLSM practical, since it allows each of those differently sized subnets to use whatever prefix length fits its needs, unlike the old classful system’s rigid boundaries. This calculator handles the CIDR side of that work directly: converting any single IP and prefix length into its mask, range, and capacity, which is the calculation you’d repeat for each subnet when planning a VLSM addressing scheme by hand.

Frequently asked questions