IP Range to CIDR Converter

Convert an address range into the smallest set of CIDR blocks that covers it exactly, or expand a CIDR block back into its first and last address. Works for IPv4 and IPv6.

What the two notations mean

An IP range is a continuous run of addresses written as a start and an end, for example 192.168.0.0 - 192.168.0.255. WHOIS records and abuse reports usually use this form.

CIDR notation combines a network address with a prefix length, for example 192.168.0.0/24. Routers and firewalls use this form because it encodes the block size in the address itself.

The two are not interchangeable. A CIDR block always starts on a power-of-two boundary, so an arbitrary range often needs several blocks to cover it exactly. Entering 192.168.0.5 - 192.168.0.20 returns five blocks rather than one, which is the correct minimal cover for that range.

Examples

• 192.168.0.0/24 is 192.168.0.0 - 192.168.0.255, 256 addresses
• 10.0.0.0/8 is 10.0.0.0 - 10.255.255.255, 16,777,216 addresses
• 2001:db8::/32 is an IPv6 prefix holding 2⁹⁶ addresses

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