Free CIDR & Subnet Calculator - IPv4 & IPv6
Enter any CIDR range to instantly get the network address, subnet mask, broadcast address, usable host range, wildcard mask, and AWS/GCP reserved host counts - for both IPv4 and IPv6. All calculations run privately in your browser; no data leaves your device.
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Last updated: May 22 2026
Reviewed by Quicktooly Team
Subnet Calculator Guide
What Is a Subnet Calculator?
A subnet calculator takes an IP address in CIDR notation (e.g. 192.168.1.0/24) and computes the network address, subnet mask, broadcast address, wildcard mask, and usable host range. It saves network engineers and cloud architects from manual binary arithmetic and is essential for VPC design, firewall rules, and IP address planning.
Why Use This CIDR Subnet Calculator?
- IPv4 & IPv6 support: Works with both address families - enter any CIDR range and get full details instantly.
- Cloud-aware host counts: Shows AWS and GCP usable host counts, accounting for their reserved IP addresses.
- Runs in your browser: No server calls - all calculations happen locally, keeping your network info private.
- Copy any value: Click any result to copy it directly to your clipboard.
- Edge cases handled: Correctly handles /31 (point-to-point), /32 (host routes), /0 (default route), and full IPv6 ranges.
- 100% free: No account, no watermark, no rate limits.
What is CIDR Notation?
CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network prefix. It takes the form IP/prefix, where the prefix length indicates how many bits are used for the network portion. For example, 192.168.1.0/24 describes a network with 256 total addresses (2⁸ host bits), of which 254 are usable by hosts.
CIDR replaced the older classful addressing system (Class A/B/C) in 1993, enabling more efficient allocation of the IPv4 address space. Today it is universally used in routing tables, firewall rules, VPC configurations, and network documentation.
AWS & GCP Reserved IP Addresses
Cloud providers reserve several IP addresses within each subnet, reducing the number available to your workloads:
- AWS reserves 5 addresses per subnet: network address, VPC router, DNS server, future use, and broadcast address.
- GCP reserves 4 addresses per subnet: network address, default gateway, second-to-last address (Google's use), and broadcast address.
This matters when sizing subnets - a /28 gives you 16 total IPs but only 11 usable hosts on AWS (16 − 5) and 12 on GCP (16 − 4).
How to Use This Subnet Calculator
- Type your CIDR range into the input field (e.g.
10.0.0.0/16or2001:db8::/48) - Click "Calculate Subnet" or press Enter to compute results
- Review the output grid - each field shows a specific subnet property
- Click any value to copy it to your clipboard instantly
Common Subnet Reference
| CIDR | Subnet Mask | Total Hosts | Usable Hosts | AWS Usable |
|---|---|---|---|---|
| /16 | 255.255.0.0 | 65,536 | 65,534 | 65,531 |
| /20 | 255.255.240.0 | 4,096 | 4,094 | 4,091 |
| /21 | 255.255.248.0 | 2,048 | 2,046 | 2,043 |
| /22 | 255.255.252.0 | 1,024 | 1,022 | 1,019 |
| /23 | 255.255.254.0 | 512 | 510 | 507 |
| /24 | 255.255.255.0 | 256 | 254 | 251 |
| /25 | 255.255.255.128 | 128 | 126 | 123 |
| /26 | 255.255.255.192 | 64 | 62 | 59 |
| /27 | 255.255.255.224 | 32 | 30 | 27 |
| /28 | 255.255.255.240 | 16 | 14 | 11 |
| /29 | 255.255.255.248 | 8 | 6 | 3 |
| /30 | 255.255.255.252 | 4 | 2 | N/A |
Frequently Asked Questions
What does /24 mean in CIDR notation?
A /24 prefix means the first 24 bits of the address identify the network, leaving 8 bits for hosts. This yields 256 total addresses (2⁸) and 254 usable host addresses (subtracting network and broadcast). It corresponds to the subnet mask 255.255.255.0.
How many IPs does AWS reserve per subnet?
AWS reserves 5 IP addresses in every VPC subnet: the network address (.0), the VPC router (.1), the DNS resolver (.2), a reserved address for future use (.3), and the broadcast address (last IP). The minimum usable subnet in AWS is a /28, which provides 11 usable addresses.
Does this calculator support IPv6?
Yes. Enter any IPv6 CIDR such as 2001:db8::/32 and the calculator will return the network address, last address, prefix length, and total address count. IPv6 subnets are astronomically large - a /64 alone contains 18,446,744,073,709,551,616 addresses.
What is the difference between /31 and /30?
A /30 gives 4 addresses (2 usable), traditionally used for point-to-point links. A /31 (RFC 3021) gives only 2 addresses with no network or broadcast - both IPs are usable, making it slightly more efficient for router-to-router links. Many modern network devices support /31 for this purpose.
Is my IP data sent to any server?
No. All calculations run entirely in your browser using JavaScript. No data is transmitted - your IP ranges, network designs, and CIDR inputs never leave your device.
What is a wildcard mask?
A wildcard mask is the bitwise inverse of a subnet mask. It is used in Cisco ACLs and OSPF configurations to specify which bits of an address are significant. For a /24 subnet (mask 255.255.255.0) the wildcard mask is 0.0.0.255.
What is a subnet mask?
A subnet mask is a 32-bit number that divides an IP address into network and host portions. It is written in dotted-decimal notation (e.g. 255.255.255.0) or as a CIDR prefix length (e.g. /24). Bits set to 1 identify the network; bits set to 0 identify the host portion.
How do I calculate the number of hosts in a subnet?
Subtract the prefix length from 32 (for IPv4) to get the number of host bits, then raise 2 to that power. For a /24, that is 2⁸ = 256 total addresses. Subtract 2 for the network and broadcast addresses to get 254 usable hosts. For cloud environments, subtract additional reserved IPs: 5 for AWS, 4 for GCP.
What is the difference between /16 and /24?
A /16 uses 16 bits for the network, leaving 16 bits for hosts - 65,536 total addresses (65,534 usable). A /24 uses 24 bits for the network, leaving 8 bits for hosts - 256 total addresses (254 usable). A /16 is typically used for an entire VPC or campus network, while a /24 is a common per-team or per-service subnet.