What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation combines an IP address with a prefix length (e.g. /24) to describe a network block. The prefix length tells you how many bits are used for the network portion.

How is the subnet mask calculated from a prefix length?

For a /24 prefix, the top 24 bits are all 1s and the remaining 8 bits are 0s, giving a subnet mask of 255.255.255.0. This calculator performs that bit math automatically.

What is the wildcard mask?

The wildcard mask is the bitwise inverse of the subnet mask. A /24 subnet mask of 255.255.255.0 yields a wildcard mask of 0.0.0.255. It is commonly used in Cisco ACLs and OSPF configurations.

How many usable hosts does a /24 subnet have?

A /24 subnet has 256 total addresses (2^8). Subtract 2 — one for the network address and one for the broadcast address — leaving 254 usable host addresses.

What are RFC 1918 private address ranges?

RFC 1918 defines three private IPv4 ranges not routed on the public internet: 10.0.0.0/8 (Class A), 172.16.0.0/12 (Class B), and 192.168.0.0/16 (Class C).

Subnet / CIDR Calculator

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Free IPv4 subnet calculator. Enter any CIDR address (e.g. 192.168.1.10/24) to instantly see the network address, broadcast address, first and last usable hosts, dotted-decimal subnet mask, wildcard mask, total addresses, usable host count, and network class. Includes quick-pick prefix presets from /8 to /30. All computation is 100% client-side — no data leaves your browser.

IPv4 CIDR Address

Quick prefix

Network

Network Address

192.168.1.0

Broadcast Address

192.168.1.255

First Usable Host

192.168.1.1

Last Usable Host

192.168.1.254

Network Class

Mask & Count

Subnet Mask

255.255.255.0

Prefix Length

/24

Wildcard Mask

0.0.0.255

Total Addresses

256

Usable Hosts

254

Frequently Asked Questions

References

RFC 4632 — Classless Inter-domain Routing — IETF
RFC 1918 — Address Allocation for Private Internets — IETF

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Frequently Asked Questions

References and sources

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