Enter an IPv4 or IPv6 address with CIDR prefix (e.g., 192.168.1.0/24).
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Click Calculate or press Enter to see subnet details.
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Use the Split panel to divide the network into smaller subnets.
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Click any value to copy it to clipboard. Explore the binary breakdown for learning.
What is CIDR Calculator / IP Subnet Calculator?
CIDR Calculator computes all properties of an IP network given a CIDR notation (e.g., 192.168.1.0/24): network address, broadcast address, usable host range, total hosts, usable hosts, subnet mask, wildcard mask, and binary representation of each. It supports both IPv4 and IPv6, and can subnet a network into smaller blocks by specifying the required number of subnets or hosts per subnet.
CIDR (Classless Inter-Domain Routing) notation expresses an IP address and its associated routing prefix as a single string: IP_address/prefix_length. The prefix length (e.g., /24) indicates how many leading bits are the network portion. /24 = 255.255.255.0 = 256 addresses (254 usable; network and broadcast addresses are reserved). /16 = 65,536 addresses. /32 = a single host. /0 = the entire internet.
Common subnets: /8 = Class A (16 million hosts), /16 = Class B (65,534 hosts), /24 = Class C (254 hosts), /30 = 2 usable hosts (point-to-point links), /32 = single host (loopback, static route). Private ranges (RFC 1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. AWS VPC default: 172.31.0.0/16. Docker default bridge: 172.17.0.0/16.
How to Use
Enter an IP address and subnet mask in CIDR notation (e.g., 10.0.0.0/16).
View the computed network address, broadcast address, and usable host range.
Use the subnet splitter to divide the network into N equal subnets of the appropriate size.
Toggle 'IPv6 mode' for IPv6 CIDR calculations (e.g., 2001:db8::/32).
Use the 'Find subnet for N hosts' calculator to get the minimum prefix length for your host count.
Examples
AWS VPC subnet planning
Result: 10.0.0.0/16 → 65,534 usable IPs; /20 subnets give 4,094 hosts each; 16 subnets per /16
Result: Is 192.168.1.100 in 192.168.1.0/25? Broadcast is 192.168.1.127 → Yes
Frequently Asked Questions
What is the difference between a network address and a broadcast address?
The network address is the first address in a subnet — all host bits are 0. It identifies the subnet but cannot be assigned to a host. The broadcast address is the last address — all host bits are 1. Traffic sent to the broadcast address is delivered to all hosts in the subnet. Between these two, the usable host range sits: 254 hosts for a /24 (256 - 2 reserved).
How do I split a /16 into multiple /24 subnets?
A /16 contains 256 /24 subnets. To subnet 10.0.0.0/16 into /24 blocks: 10.0.0.0/24, 10.0.1.0/24, ..., 10.0.255.0/24. Each /24 has 256 addresses (254 usable). The CIDR calculator's subnet splitter automates this — specify your /16 and the desired /24 size, and it lists all 256 subnets.
What is VLSM (Variable Length Subnet Masking)?
VLSM allows subnets within a network to have different sizes (prefix lengths). Instead of dividing 10.0.0.0/16 into equal /24 blocks, you can allocate a /24 for a large office (254 hosts), a /28 for a small office (14 hosts), and a /30 for a router link (2 hosts). VLSM maximizes address utilization but requires careful planning to avoid overlapping address spaces.
How does IPv6 CIDR work?
IPv6 uses the same CIDR notation but with 128-bit addresses (vs. 32-bit IPv4). A /64 is the standard allocation for a single subnet (last 64 bits are the interface identifier). A /48 is a common ISP customer allocation (allows 65,536 /64 subnets). A /32 is a typical ISP prefix. The math is the same as IPv4 but the address space is incomprehensibly larger: 2^128 ≈ 3.4 × 10^38 addresses.
What are the reserved IPv4 address ranges?
Private (RFC 1918): 10.0.0.0/8, 172.16.0.0/12 (172.16-31.x.x), 192.168.0.0/16. Loopback: 127.0.0.0/8 (127.0.0.1 most common). Link-local (APIPA): 169.254.0.0/16. Documentation/examples: 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24. Multicast: 224.0.0.0/4. Broadcast: 255.255.255.255/32. These ranges are not routable on the public internet.