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CIDR to Subnet Mask Converter

CIDR Calculator

Convert any CIDR prefix notation (/0 to /32) into dotted decimal subnet mask, wildcard mask, usable host count, and network class equivalence instantly.

Leave as base IP or enter your interface IP

254 Usable Hosts
/0/8/16/24/32
Common Prefixes:
Subnet Mask
255.255.255.0
Dotted decimal netmask
Wildcard Mask
0.0.0.255
Cisco ACL / OSPF inverse mask
Usable Hosts
254
Total assignable host interfaces
Total Addresses
256
2^(32 - CIDR) addresses
CIDR Notation:10.0.0.0/24
Network Range:10.0.0.0 – 10.0.0.255
Class Equivalent:1 Class C
Binary Netmask:11111111.11111111.11111111.00000000

Understanding CIDR Notation

In modern IP networks, Classless Inter-Domain Routing (CIDR) replaced the historic Class A, B, and C architecture. The forward slash notation (such as /24 or /16) explicitly states how many bits belong to the network prefix. The remaining bits (32 minus prefix length) belong to the host field.

For instance, an enterprise AWS VPC or Azure Virtual Network typically receives a /16 allocation (yielding 65,534 assignable host addresses). Network engineers then subdivide that /16 into multiple /24 or /20 subnets corresponding to Public Subnets, Private Application Subnets, and Database Subnets across distinct Availability Zones.

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Frequently Asked Questions

Clear answers to common questions about subnetting, CIDR notation, and network design.

What does CIDR mean in networking?
CIDR stands for Classless Inter-Domain Routing. Introduced in 1993 (RFC 1519) to replace the rigid Class A, B, and C address system, CIDR allows routers to group IP addresses into variable-sized subnets using prefix notation like /24, conserving IPv4 space and reducing the size of global routing tables.
How do I convert a CIDR prefix to a Subnet Mask?
A CIDR prefix indicates the number of leading binary 1s in a 32-bit subnet mask. For example, /24 has twenty-four 1s followed by eight 0s: 11111111.11111111.11111111.00000000. Converting each 8-bit octet to decimal gives 255.255.255.0.
What is the largest and smallest CIDR block?
In IPv4, the largest block is /0 (0.0.0.0/0, representing all 4,294,967,296 IPv4 addresses on the Internet). The smallest block is /32, representing a single host IP address with 1 usable host.
Why does /24 have 254 usable hosts and not 256?
In any standard IPv4 subnet, the first address (all host bits 0) is reserved as the Network ID, and the last address (all host bits 1) is reserved as the Broadcast Address. Therefore, 256 total addresses minus 2 reserved addresses equals 254 usable host addresses.
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