CIDR Calculator

Calculate subnet information from an IP address with CIDR notation. Supports IPv4 and IPv6.

CIDR Input
Clear
Network Information
Network Address 2001:db8:42::
Broadcast Address Not used in IPv6
Subnet Mask /48
Wildcard Mask Not used in IPv6
CIDR Notation 2001:db8:42::/48
IP Class IPv6
Host Information
First Host 2001:db8:42::
Last Host 2001:db8:42:ffff:ffff:ffff:ffff:ffff
Total Hosts 1,208,925,819,614,629,174,706,176
Usable Hosts 1,208,925,819,614,629,174,706,176
Workflow Summary
Planning Notes
1. IPv6 uses prefix scope rather than broadcast addresses.
Planning Presets
Home lab / office LAN
192.168.10.0/24
Size a private LAN for infrastructure, servers, and user devices without leaving the RFC 1918 range.
VPC app subnet
10.24.8.0/25
Check a cloud subnet size before you carve out routing, NAT, or access-control rules.
IPv6 block planning
2001:db8:42::/48
Review an IPv6 prefix before you hand a routed block to multiple services or sites.
What is CIDR?

CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing Internet Protocol packets. Introduced in 1993, CIDR replaced the previous classful network addressing architecture. CIDR notation appends a suffix indicating the number of bits in the network prefix (e.g., 192.168.1.0/24), allowing flexible subnetting beyond the traditional Class A, B, C boundaries.

How CIDR Works

CIDR uses a prefix length to define the boundary between the network and host portions of an IP address. The prefix length (e.g., /24) indicates how many bits are used for the network address. The remaining bits are available for host addresses. A /24 prefix means 24 bits for the network (256 total addresses, 254 usable hosts), while /16 provides 65,536 total addresses.

Common Use Cases
  • Planning IP address allocation for networks
  • Configuring firewall rules and access control lists
  • Designing VPC subnets in cloud environments
  • Calculating available hosts for network segments
  • Optimizing routing table entries with route aggregation
Examples
Input: 192.168.1.0/24
Output: 254 usable hosts, mask 255.255.255.0
Input: 10.0.0.0/8
Output: 16,777,214 usable hosts, mask 255.0.0.0
Input: 172.16.0.0/12
Output: 1,048,574 usable hosts, mask 255.240.0.0
Frequently Asked Questions

What is the difference between /24 and /25?

A /24 network has 256 total addresses (254 usable hosts) with subnet mask 255.255.255.0. A /25 network splits this in half: 128 total addresses (126 usable hosts) with subnet mask 255.255.255.128.

Why are 2 addresses subtracted for usable hosts?

The first address in a subnet is the network address and the last is the broadcast address. These cannot be assigned to hosts, so they are subtracted from the total to get usable host count. Exception: /31 and /32 subnets have special rules (RFC 3021).

What are private IP address ranges?

RFC 1918 defines three private ranges: 10.0.0.0/8 (Class A), 172.16.0.0/12 (Class B), and 192.168.0.0/16 (Class C). These addresses are not routable on the public internet and are used for internal networks.