Free CIDR Calculator – Expand, Split, Overlap Check & Summarize

What Is CIDR Notation and How This Calculator Helps

CIDR (Classless Inter-Domain Routing, defined in RFC 4632) replaced the old Class A/B/C addressing system by allowing subnet boundaries at any bit position, not just on octet boundaries. A CIDR block like 10.0.0.0/16 tells you the first 16 bits are the network prefix and the remaining 16 bits enumerate hosts, giving 65,536 addresses in total. This CIDR calculator takes a notation like that and expands it into its full address range (first IP to last IP), shows total address count, and provides four operational modes: expand, split, overlap-check, and summarize. Network engineers designing routing tables, cloud architects planning AWS VPC or Azure VNet allocations, and firewall administrators writing prefix-list rules all work in CIDR daily. The expand mode takes one CIDR block and outputs the first address (network), last address, total count of addresses, and the equivalent subnet mask. The split mode divides a single CIDR into smaller equally-sized subnets at a new longer prefix length — for example splitting a /24 into four /26 blocks — showing each resulting sub-block's range. The overlap-check mode takes two CIDR blocks and determines whether they share any addresses, which is essential when assigning non-overlapping ranges to peered VPCs or avoiding conflicts in routing tables. The summarize mode takes two or more CIDR blocks and finds the single shortest-prefix supernet that covers all of them — route summarization reduces the size of routing tables and is a core technique in BGP and OSPF network design. Expanding 192.168.1.0/24 yields first address 192.168.1.0, last address 192.168.1.255, and a total of 256 addresses. Splitting that same /24 into /26 subnets gives four blocks: 192.168.1.0/26 (0–63), 192.168.1.64/26 (64–127), 192.168.1.128/26 (128–191), and 192.168.1.192/26 (192–255), each containing 64 addresses. Checking overlap between 10.0.0.0/16 and 10.0.5.0/24 returns OVERLAP because the /24 sits entirely inside the /16. Summarizing 192.168.0.0/24 and 192.168.1.0/24 yields 192.168.0.0/23 — one route covering both. The tool supports IPv6 as well, using BigInt arithmetic for the full 128-bit address space; expanding 2001:db8::/32 shows the first and last addresses across 2^96 possible interface IDs. Cloud engineers use the split mode when creating subnet allocations inside a VPC: AWS requires non-overlapping CIDRs within a VPC, and splitting the VPC's master /16 into /20 availability-zone blocks and then further into /24 subnet blocks is a standard design pattern. Network operators performing route summarization in BGP check that a proposed aggregate actually covers all the component prefixes without leaking coverage to addresses they don't own — the summarize mode confirms this visually. Security teams running overlap checks prevent accidental "permit" ACL entries for a CIDR that bleeds into a restricted range, a mistake that opens unintended access paths. Remember that CIDR prefix lengths represent powers of two in address count: each one-bit increase in the prefix halves the number of addresses in the block, and each one-bit decrease doubles it. When splitting, the new prefix must be longer (more specific) than the original, and when summarizing, the resulting prefix will be shorter (less specific). IPv6 subnets conventionally use /64 for the interface portion (RFC 4291), so splitting below /64 is unusual in practice even though the tool supports it mathematically. A subtle pitfall when summarizing is assuming any two blocks can merge into a clean supernet — they can only aggregate if they are adjacent and aligned on the right bit boundary, so 192.168.0.0/24 and 192.168.2.0/24 do not summarize into a single /23 the way 192.168.0.0/24 and 192.168.1.0/24 do. The overlap check guards against the opposite mistake, catching a proposed allocation that unintentionally sits inside a range you already use. All calculations run entirely in your browser using 32-bit unsigned arithmetic for IPv4 and BigInt for IPv6; no address data is transmitted anywhere, so the tool is safe to use on internal and production addressing plans.

CIDR Calculator Formula & Method

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