"/24" next to an IP address looks cryptic until you know what it's counting — once it clicks, subnetting stops being a memorization exercise and becomes simple arithmetic.

What CIDR Notation Actually Counts

The number after the slash (e.g., /24) is the number of bits reserved for the network portion of the address, out of 32 total bits in an IPv4 address — the remaining bits identify individual hosts on that network. A /24 leaves 8 bits for hosts; a /16 leaves 16 bits.

Common Prefixes and What They Mean

CIDRSubnet MaskUsable Hosts
/24255.255.255.0254
/25255.255.255.128126
/28255.255.255.24014
/16255.255.0.065,534

Why Break a Network Into Subnets

Isolation and organization: subnetting divides one large network into smaller, manageable segments — separating departments, limiting broadcast traffic, and isolating parts of a network for security are all reasons to subnet rather than run everything on one flat network.

Network Address, Broadcast Address, and Usable Range

Within any subnet, the first address is reserved as the network address and the last as the broadcast address — neither is assignable to a device. The usable host range is always two less than the total addresses in the subnet, which is why a /24 (256 total addresses) has 254 usable hosts, not 256.

Step-by-Step: Calculate Subnet Details

  1. Enter an IP address and CIDR prefix
  2. Get the subnet mask, network address, broadcast address, and usable host range instantly

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