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CIDR to Subnet Mask Calculator

Enter the CIDR prefix length to get its subnet mask.

How to use

  1. Enter your values in the fields above.
  2. Press Calculate to see your result instantly.
  3. Use the Share button to copy a link to your result.

About this calculator

CIDR notation (Classless Inter-Domain Routing) expresses a subnet mask as a slash followed by the number of leading 1-bits — /24 means the first 24 bits of the 32-bit address are the network portion. This converts a prefix length directly into the dotted-decimal subnet mask everyone actually configures on routers and servers (/24 = 255.255.255.0), plus the wildcard mask (its bitwise inverse, used in Cisco ACLs) and the count of usable host addresses.

CIDR replaced the old rigid Class A/B/C system in the 1990s to make IP address allocation far more efficient, and converting between prefix length and subnet mask is a daily task for network administrators configuring VLANs, firewalls, and routing tables, and a core topic in every networking certification syllabus.

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