Network calculators

CIDR calculator

Convert between CIDR notation and IP address ranges — useful for firewall rules, allow-lists and cloud networking.

CIDR → IP range

First address
203.0.113.0
Last address
203.0.113.63
Addresses
64

IP range → CIDR blocks

The smallest set of CIDR blocks covering exactly this range (6 blocks):

  • 10.0.0.5/32
  • 10.0.0.6/31
  • 10.0.0.8/29
  • 10.0.0.16/28
  • 10.0.0.32/29
  • 10.0.0.40/32

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) writes a network as an address plus a prefix length, such as 203.0.113.0/26. The prefix says how many leading bits are fixed; a /26 contains 2^(32−26) = 64 addresses.

Why ranges become several blocks

A CIDR block always starts on a boundary that is a multiple of its size. An arbitrary range such as 10.0.0.5 – 10.0.0.40 doesn't line up with those boundaries, so it has to be expressed as several blocks. The calculator returns the smallest possible set that covers the range exactly — no extra addresses.

Questions

Does this support IPv6?

Not yet — this calculator handles IPv4. IPv6 prefixes follow the same principle with 128-bit addresses.