CIDR Calculator – IP Range to CIDR

How to Use the Calculator

1. Choose a Mode: "CIDR → address range", "Address range → CIDR blocks", or "Aggregate a list of CIDRs". Only the inputs used by the selected mode are shown.
2. In CIDR → address range mode, enter a CIDR block such as 10.0.0.0/22. A bare address like 192.168.1.1 is treated as a /32 (a single host). The results show the first address (network), last address (broadcast), total addresses, netmask, wildcard mask, and the canonical block.
3. In Address range → CIDR blocks mode, enter the First IP address and Last IP address of the range (both inclusive). The calculator returns the minimal number of CIDR blocks that cover the range exactly, with no gaps or overlaps, plus a per-block breakdown.
4. In Aggregate a list of CIDRs mode, paste your blocks into the CIDR blocks field — one per line, though commas and semicolons also work. Overlapping and adjacent blocks are merged automatically, and the whole list is summarized as the smallest exact set of blocks.
5. Check Calculation details under the results for the step-by-step logic, including a note when an entered address had host bits set. For planning subnets around one block, the IP subnet calculator covers the same operations from a subnetting angle.

What Is CIDR?

Classless Inter-Domain Routing (CIDR) replaced the old class A/B/C system with a flexible notation that writes an address and its prefix length together: a.b.c.d/p. The prefix \(p\) says how many leading bits are common to every address in the block; the remaining \(32-p\) bits vary freely, so the block contains \(2^{32-p}\) addresses. That single rule explains everything else: a /24 has \(2^8 = 256\) addresses and netmask 255.255.255.0, a /22 has \(2^{10} = 1024\) addresses and netmask 255.255.252.0, and a /32 is one exact address.

A key consequence of the bit structure is alignment: a block of \(2^k\) addresses can only start at an address that is a multiple of \(2^k\). This is why, for example, 192.168.1.16/28 is valid (16 addresses starting at a multiple of 16) but 192.168.1.8/28 is not a proper block boundary. When you convert an arbitrary first–last range to CIDR, the range rarely lines up with a single block, so it must be split. The calculator uses a greedy algorithm: at each step it takes the largest block that both fits in the remaining addresses and is correctly aligned, which provably yields the minimum possible number of blocks.

Interpreting the numbers matters in practice. "Total addresses" is the raw size of the block, but classic subnetting (prefixes /0 through /30) reserves the first address as the network identifier and the last as the broadcast address, leaving two fewer usable for hosts. A /31 (per RFC 3021, for point-to-point links) and a /32 (a single host) are exceptions where every address is usable. If your goal is to plan how a larger block divides into smaller subnets with hosts per subnet, use the subnet calculator; to verify address arithmetic by hand, the binary converter shows the bit patterns behind the masks.

Tips and Limitations

• The calculator handles IPv4 only (four octets, 0–255, prefix /0–/32). IPv6 addresses are rejected.
• If you enter an address with host bits set, such as 10.0.1.7/22, the results describe the containing block (10.0.0.0/22), and the calculation details flag this. To be explicit, always enter the network address.
• A first–last range is converted to the minimal exact set of blocks — coverage with no gaps and no duplicates. This is usually what routing tables and access-control lists want.
• Aggregation merges blocks only when they overlap or are directly adjacent; a gap of even one address keeps them separate.
• Very large results (e.g. aggregating many scattered /32s) can run to thousands of blocks; the table shows the first 200 and reports the full count.

Common CIDR Prefixes Reference

Each extra bit in the prefix halves the block; each bit removed doubles it. The relationship \(N = 2^{32-p}\) is all you need to reconstruct any row of this table.