Subnet / CIDR Calculator

How to Calculate Subnet Ranges & CIDR Notation

1. Enter an IP address and select a CIDR prefix length (e.g. 192.168.1.1/24 or 10.0.0.0/16).
2. Inspect the calculated subnet mask, network address, IPv4 broadcast address (if applicable), and first/last usable host addresses.
3. View the 32-bit binary breakdown (IPv4) highlighting network bits versus host bits.
4. Use the Subnet Splitter (VLSM) tab to divide parent networks into smaller child subnets.
5. Use the CIDR Supernetting tab to aggregate multiple subnets into a single summarized route.
6. Copy full formatted reports or individual network properties to your clipboard instantly.

Key Features

• Comprehensive IPv4 Subnetting: Instant calculation of Network Address, IPv4 Broadcast Address, First Usable Host, Last Usable Host, Wildcard Mask, and Total/Usable Hosts.
• 32-Bit Binary Visualizer (IPv4): Visual bitmask breakdown highlighting network bits versus host bits across all four octets for IPv4 addresses.
• VLSM Subnet Splitter: Divide parent CIDR blocks into customized child subnets with capacity counts and usable ranges.
• CIDR Supernet Summarizer: Determine the smallest covering CIDR prefix (supernet) for arrays of IP addresses or subnets, noting that summarizing non-contiguous networks may include additional address space.
• IPv6 Subnet Calculator: Full support for IPv6 address expansion, compression, prefix lengths ((/0) to (/128)), and scope categorization to identify address types such as Global Unicast, Unique Local, Link-Local, Multicast, Loopback, and Unspecified.
• 100% Privacy-First Execution: All calculations are performed entirely in local browser memory with zero network requests.

Common Use Cases

• Cloud Infrastructure Design: Plan AWS VPCs, GCP Subnets, and Azure VNets with precise non-overlapping IP address ranges.
• Network Administration: Configure router interfaces, firewalls, ACLs, and DHCP pools with verified wildcard masks.
• IP Address Allocation (IPAM): Split corporate networks into departmental subnets using Variable Length Subnet Masking (VLSM).
• Route Summarization: Aggregate multiple routing table entries into compact supernet prefixes to reduce routing table size and simplify route advertisements.

Frequently Asked Questions

What is CIDR Notation?

Classless Inter-Domain Routing (CIDR) notation expresses an IP address and its associated network prefix length separated by a slash (e.g. 192.168.1.0/24). The number after the slash represents the number of bits assigned to the network prefix, while the remaining bits ((32 - \text{prefix}) for IPv4) are allocated to host addresses. Note that a /32 prefix represents a single host route.

How are usable hosts calculated?

For a standard IPv4 subnet with prefix length (P), total IP addresses equal (2^{32 - P}). Usable hosts are calculated as (2^{32 - P} - 2) because the first address is reserved as the Network Address and the last address is reserved as the IPv4 Broadcast Address. Note that for (/31) point-to-point links (RFC 3021), both addresses are usable hosts.

What is a Wildcard Mask?

A wildcard mask is the bitwise inverse of a subnet mask. For valid IPv4 subnet masks, subtracting each octet of the subnet mask from 255 produces the same result (e.g., 255.255.255.0 inverted yields 0.0.0.255). Wildcard masks are widely used in router Access Control Lists (ACLs) and OSPF configurations to specify which IP bits to match or ignore.

Why does IPv6 have no broadcast address?

IPv6 eliminated broadcast addresses to improve network efficiency and reduce unnecessary background processing on host devices. Instead of broadcasting packets to every host on a local network segment, IPv6 relies on targeted multicast addresses (such as solicited-node multicast addresses) and anycast addressing.

What is the difference between a subnet mask and a CIDR prefix?

A subnet mask is a 32-bit value expressed in dotted-decimal format (e.g., 255.255.255.0) that separates network bits from host bits. A CIDR prefix represents the same mask concisely by specifying the count of leading 1-bits after a slash (e.g., /24).

What is Variable Length Subnet Masking (VLSM)?

Variable Length Subnet Masking (VLSM) allows network engineers to divide an IP address space into subnets of varying prefix lengths rather than fixed-size blocks. This optimizes IP address utilization by tailoring subnet allocations directly to the actual host capacity required.

What is route summarization (supernetting)?

Route summarization, or supernetting, combines multiple contiguous or related subnets into a single, broader CIDR prefix. This consolidation reduces routing table sizes and simplifies route advertisements across network boundaries.

Where can I learn more about network terminology and subnetting?

For a comprehensive glossary of network concepts, protocols, and CIDR math, read The Complete Networking Glossary for Backend Engineers and Demystifying Subnetting, CIDR & IP Addressing.