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CIDR Notation Converter

Convert between CIDR prefixes, dotted subnet masks and wildcard masks, for IPv4 or IPv6.

NETWORK · IN YOUR BROWSER

Runs in your browser; nothing you type leaves this page. Too long to share as a link.

HISTORY

Your last 5 runs appear here for 7 days, on this device only.

RESULT

Fill in the form and run the tool; the result appears here.

CIDR Notation Converter translates between CIDR notation, subnet mask and wildcard mask in your browser. Give it a block such as 192.168.1.0/24, a bare prefix such as 24 or a dotted mask such as 255.255.255.0; with an address in the block it also shows the network, broadcast and address count.

How to read the result

The output shows each equivalent form of the same mask side by side. A dotted mask whose 1 bits are not all leading (255.255.0.255) is rejected, because no prefix length can express it. The tool converts one block at a time; it does not split or merge ranges.

Defender tips

  • Remember that a Cisco ACL wildcard mask is the inverse of a subnet mask (0.0.0.255 matches a /24, not 255.255.255.0); this is a frequent source of ACLs that accidentally match far more or less than intended.
  • Use the smallest CIDR block that actually covers your intended range in a rule; an oversized block in a firewall or ACL rule is a common way access ends up broader than the design intended.

FAQ

Why do wildcard masks look inverted compared to subnet masks?

Cisco IOS ACLs historically specify which bits to ignore (the wildcard) rather than which bits define the network (the subnet mask), so a /24's wildcard mask is the bitwise complement of its subnet mask.

What does supernetting mean?

Combining several contiguous, same-size smaller blocks into one larger CIDR block for routing efficiency, the reverse of subnetting a large block into smaller ones.

Does it handle IPv6 prefixes?

Yes. A prefix such as 2001:db8::/32 gives the mask in hextet form, the network, the last address and the address count, and a contiguous IPv6 mask converts back to its prefix. There is no wildcard mask for IPv6.