Krill KitsKrill Kits// A swarm of small, sharp tools for letters, numbers, and units.
§ 01 / TOOL

CIDR Calculator.

STATUS READYSCHEME IPv4MAX PREFIX /32
> CIDR
FORMAT IP/PREFIX

Subnet.

254 USABLE
  • NETWORK
    192.168.1.0
  • BROADCAST
    192.168.1.255
  • MASK
    255.255.255.0
  • WILDCARD
    0.0.0.255
  • FIRST HOST
    192.168.1.1
  • LAST HOST
    192.168.1.254
  • TOTAL ADDRESSES
    256
  • USABLE HOSTS
    254
// BINARY
NETWORK11000000.10101000.00000001.00000000
MASK11111111.11111111.11111111.00000000
§ 02 / ABOUT

How CIDR works.

CIDR (Classless Inter-Domain Routing) replaced classful addressing in the 1990s. The prefix tells you how many bits identify the network — the rest identify hosts within it. So /24 means “the first 24 bits are network, last 8 are hosts” → 256 total addresses, 254 usable.

// PREFIX REFERENCE

  • /8 — 16,777,216 total / 16,777,214 usable (Class A size)
  • /16 — 65,536 / 65,534 (Class B)
  • /24 — 256 / 254 (Class C / typical home network)
  • /27 — 32 / 30
  • /28 — 16 / 14
  • /30 — 4 / 2 (small point-to-point)
  • /31 — 2 / 2 (RFC 3021 point-to-point, no broadcast)
  • /32 — 1 / 1 (single host)

// THE BINARY VIEW

The BINARY section colours network bits in neon and host bits in gray. This is the “why” behind every other number — masking is just an AND between the address and the mask.

§ 03 / FAQ

CIDR questions.

What does /24 mean?+
The first 24 bits of the address are the network portion; the remaining 8 bits identify hosts. /24 = 256 total addresses (2^8), 254 usable hosts (network address and broadcast take two). Equivalent to subnet mask 255.255.255.0.
What’s the difference between subnet mask and wildcard mask?+
They’re inverses. Subnet mask has 1-bits for network portion, 0-bits for host portion (255.255.255.0 for /24). Wildcard mask flips that — 0.0.0.255 for /24. ACLs in Cisco IOS use wildcard masks. Modern routing usually quotes subnet masks.
Why does a /24 have 254 usable hosts instead of 256?+
The first address is the network identifier (192.168.1.0), the last is the broadcast (192.168.1.255). Neither can be assigned to a host. Special cases: /31 has 2 usable (RFC 3021 point-to-point), /32 has 1 (single host).
How do I figure out usable hosts for any prefix?+
2^(32 − prefix) − 2. So /24 → 2^8 − 2 = 254. /22 → 2^10 − 2 = 1022. /28 → 2^4 − 2 = 14. The /31 and /32 special cases skip the −2.
§ 04 / TOOLS

Related calculators.

§ 05 / READING

Deeper dives.