What is a CIDR prefix?

CIDR (Classless Inter-Domain Routing) notation writes the network size as a slash followed by the number of leading 1-bits in the subnet mask. /24 means the first 24 bits identify the network, leaving 8 bits (256 addresses) for hosts.

Is my IP address sent anywhere?

No. All math runs locally in your browser with JavaScript bitwise operations. Nothing is uploaded, and the IP you enter is not stored — only your chosen view options are remembered for convenience.

Subnet Calculator — IPv4 CIDR, Netmask & Host Range

📋

Walk-through

How to Use This Calculator

4 steps

1

Enter an IPv4 address

Type any address that sits inside the network you care about — for example 192.168.1.10. It does not have to be the network address; the calculator normalizes host bits and shows you the true network ID.

2

Choose a CIDR prefix or subnet mask

Use the slider or dropdown to pick a prefix from /0 to /32, or switch to mask mode and enter a dotted mask like 255.255.255.0. The prefix and mask stay in sync, so you can think in whichever notation you prefer.

3

Read the subnet details

The result card shows usable host count, and the grid below lists the network address, broadcast, first and last usable host, total addresses, subnet mask, wildcard mask, and IP class. Everything updates instantly as you type.

4

Inspect binary bits or split the block

Open Binary View to see exactly where the network/host boundary falls in each octet, or use Split into Subnets to break the network into smaller equal subnets (a quick VLSM helper). Copy any value, or export the whole block as CSV.

⚡

Reference

Formula & Methodology

4 formulas

Subnet mask from a CIDR prefix

mask = 0xFFFFFFFF << (32 − prefix)

The prefix is the number of leading 1-bits in the 32-bit mask. A /24 sets the first 24 bits to 1 (255.255.255.0); the remaining 8 bits are 0 and belong to the host portion.

Network and broadcast address

network = IP AND mask · broadcast = network OR (NOT mask)

Bitwise-ANDing the IP with the mask clears the host bits, giving the network address. ORing the network with the wildcard (the inverted mask) sets all host bits, giving the broadcast address.

Usable host count

usable = 2^(32 − prefix) − 2

A block of 2^(32 − prefix) addresses reserves the first (network) and last (broadcast) address, leaving the rest for hosts. Exceptions: /31 has 2 usable addresses (RFC 3021 point-to-point) and /32 is a single host route.

Wildcard mask

wildcard = NOT mask

The wildcard mask is the bitwise inverse of the subnet mask (255.255.255.0 → 0.0.0.255). Cisco ACLs and OSPF use wildcard masks to match address ranges.

📖

Glossary

Key Terms Explained

10 terms

CIDR prefix Classless Inter-Domain Routing notation: a slash followed by the number of network bits (0–32). /24 means 24 network bits and 8 host bits.

Network address The first address in a subnet, with all host bits set to 0. It identifies the subnet itself and cannot be assigned to a host.

Broadcast address The last address in a subnet, with all host bits set to 1. Traffic sent here reaches every host on the subnet; it cannot be assigned to a single host.

Subnet mask A 32-bit value of contiguous 1-bits (network) followed by 0-bits (host), e.g. 255.255.255.0. It marks the boundary between the network and host portions of an address.

Wildcard mask The bitwise inverse of the subnet mask. Used in Cisco access control lists and OSPF to specify which address bits must match.

Usable hosts The addresses in a subnet that can be assigned to devices: 2^(32 − prefix) − 2 for most blocks, with /31 and /32 as special cases.

VLSM Variable Length Subnet Masking: dividing a network into subnets of different sizes by using different prefix lengths, instead of one fixed mask everywhere.

RFC 1918 (private addresses) The ranges 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 reserved for private networks. They are not routable on the public internet and are typically used behind NAT.

RFC 3021 (/31 links) A standard that allows a /31 to be used on point-to-point links, making both addresses usable instead of reserving a network and broadcast address.

IP class The legacy classful scheme based on the first octet: Class A (1–126), B (128–191), C (192–223), D (224–239, multicast), E (240–255, reserved). Modern routing uses CIDR, but the class is still a useful label.

👥

Scenarios

Real-World Examples

3 worked examples

🏠

Home network

192.168.1.10 /24

IP 192.168.1.10 Prefix /24

The network is 192.168.1.0 and the broadcast is 192.168.1.255. Usable hosts run 192.168.1.1–192.168.1.254 — 254 addresses. The subnet mask is 255.255.255.0 and the wildcard is 0.0.0.255. This is a Class C, RFC 1918 private block: the standard home/office LAN.

🔗

Point-to-point link

10.10.10.0 /30 vs /31

IP 10.10.10.0 Prefix /30

A /30 gives 4 addresses and 2 usable hosts (10.10.10.1 and 10.10.10.2) — the classic router-to-router link. Switch to /31 and RFC 3021 lets you use both addresses (10.10.10.0 and 10.10.10.1), saving an address on every point-to-point link across a large network.

🧮

Splitting a block

192.168.1.0/24 → /26

IP 192.168.1.0 Prefix /24

Splitting a /24 into /26 subnets yields four equal blocks (192.168.1.0, .64, .128, .192), each with 62 usable hosts. The Split into Subnets tab lists every resulting subnet with its network, broadcast, host range, and count — handy when carving a department network into floors or VLANs.

📄

Deep Dive

Subnetting Explained: CIDR, Masks, and Host Counts

3 chapters · 4 min read

Subnetting splits a block of IP addresses into smaller networks by moving the boundary between the network and host portions of an address. This calculator does the bitwise math for you, but understanding what is happening makes network design and troubleshooting far easier.

How CIDR and subnet masks work

An IPv4 address is 32 bits, usually written as four decimal octets. A subnet mask is also 32 bits: a run of 1-bits marking the network portion, followed by 0-bits marking the host portion. CIDR notation simply counts those 1-bits — a /24 mask is 255.255.255.0, because the first 24 bits are 1.

To find the network address, the calculator ANDs the IP with the mask, clearing every host bit. To find the broadcast, it ORs the network with the wildcard mask (the inverted subnet mask), setting every host bit. The addresses in between are the usable host range.

Why you subtract 2 (and when you don't)

In a normal subnet, two addresses are reserved: the first identifies the network and the last is the broadcast address. Neither can be assigned to a device, so usable hosts = 2^(32 − prefix) − 2. A /24 has 256 addresses but only 254 usable hosts.

Two prefixes break this rule. A /31 (RFC 3021) is meant for point-to-point links where broadcast is meaningless, so both of its two addresses are usable. A /32 is a single host route — one address, no network or broadcast distinction — commonly used for loopbacks and host-specific routing.

Public vs. private ranges and address classes

RFC 1918 reserves three private ranges — 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 — that are not routable on the public internet and sit behind NAT on most home and office networks. The calculator flags whether your address is private, public, loopback (127.0.0.0/8), or link-local (169.254.0.0/16).

It also reports the legacy IP class (A–E). Classful addressing predates CIDR and no longer governs routing, but the class is still a quick sanity check and shows up in older documentation and exam questions.

❓

Questions

Frequently Asked Questions

7 questions

Why do I subtract 2 from the host count?+

The first address in a subnet is the network identifier and the last is the broadcast address; neither can be assigned to a host. So usable hosts = 2^(32 − prefix) − 2. The exceptions are /31 (RFC 3021 point-to-point, 2 usable addresses) and /32 (a single host route).

What is the difference between a subnet mask and a CIDR prefix?+

They describe the same thing two ways. The CIDR prefix counts the network bits (/24), while the dotted subnet mask spells those bits out in decimal (255.255.255.0). This calculator keeps both in sync, so you can enter whichever you prefer.

What is a wildcard mask used for?+

A wildcard mask is the bitwise inverse of the subnet mask — 255.255.255.0 becomes 0.0.0.255. Cisco access control lists and OSPF network statements use wildcard masks to match a range of addresses, where a 0 bit means 'must match' and a 1 bit means 'don't care'.

Which IP ranges are private?+

RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 for private use. These are not routable on the public internet and are commonly used behind NAT. The calculator labels your address as private, public, loopback, or link-local automatically.

What happens if I enter a host address instead of the network address?+

That is fine — enter any address inside the network. The calculator masks off the host bits and shows the true network address. It also notes when the address you typed had host bits set, so you can see the normalization.

Does this calculator support IPv6?+

No. This tool covers IPv4 only. IPv6 uses 128-bit addresses, hexadecimal notation, and different conventions (for example, there is no broadcast address), so it needs a dedicated IPv6 subnet calculator.

Is my IP address sent to a server?+

No. All subnetting math runs locally in your browser using JavaScript bitwise operations. The IP you enter is never uploaded, and it is not saved to your device — only your chosen view options (prefix and mode) are remembered for convenience.

Download a one-page PDF of your numbers instantly. Add your email to also get our occasional calculator tips — no spam, unsubscribe anytime. Privacy.

Subnet Calculator