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CIDR Overlap Checker

CIDR Overlap Checker is part of Toolspire and is ready as soon as this page loads. 10.0.0.0/8 contains 10.1.2.0/24. 10.0.0.0/24 and 10.0.1.0/24 do not touch. Real prefixes do not overlap halfway, because both are aligned blocks.

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What you can do here

Four plain answers

Same range, first contains second, second contains first, or disjoint. The detail shows both broadcast ranges.

The host bits are masked off

10.1.2.9/24 is the same block as 10.1.2.0/24. The typed host does not create a different range.

IPv4 only

The subnet calculator is the page for IPv6, VLSM and a single block’s host count. This page only compares two IPv4 prefixes.

The sequence

How to use the CIDR Overlap Checker

  1. 1

    Type two prefixes with a slash

    10.0.0.0/8 and 10.1.2.0/24 are the sample. A mask written as 255.255.255.0 is not accepted here.

  2. 2

    Read contains before you read overlap

    If the result says contains, every address of the smaller block is inside the larger one. There is no partial case to worry about.

  3. 3

    Use the subnet page for the host list

    This page does not count usable hosts or split a block. It only answers how the two ranges sit.

Worth knowing

  • CIDR Overlap Checker on Toolspire: each prefix is masked to its network address and broadcast address, then the two ranges are compared.
  • 10.0.0.0/8 contains 10.1.2.0/24. Reverse the boxes and the result is “second contains first”.
  • 10.0.0.0/24 and 10.0.1.0/24 are disjoint. Same-size neighbours do not share an address.
  • A halfway overlap is not a possible answer for two valid CIDR blocks. They nest, they match, or they sit apart. IPv6 is not compared here.

Why CIDR blocks do not overlap halfway

A CIDR prefix is a power-of-two range that starts on its own alignment. Take two of them. Either one range sits entirely inside the other, they are the same range, or they share no address. The awkward case people fear — the first half of one block colliding with the second half of another — cannot happen when both inputs are real prefixes. If a diagram shows that kind of overlap, one of the boxes is not a CIDR block. This checker masks each input, compares the two closed ranges, and uses those four words. It does not plan VLSM and it does not speak IPv6.

FAQ

Questions about CIDR Overlap Checker

Where does CIDR Overlap Checker run?

CIDR Overlap Checker runs in this tab on Toolspire. What you type or drop in is not sent to us and is not stored on a server.

Does 10.0.0.0/8 contain 10.1.2.0/24?

Yes. The result is “First contains second”. Swap the boxes and it becomes “Second contains first”.

Can two different /24 networks overlap?

No. Two different blocks of the same size are disjoint. 10.0.0.0/24 and 10.0.1.0/24 do not share an address.

What if I type 10.1.2.9/24?

The host bits are cleared. That is the block 10.1.2.0–10.1.2.255, the same as 10.1.2.0/24.

Where do I calculate hosts or IPv6?

On the IP subnet calculator. This page only compares two IPv4 prefixes.

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