Know the whole range,
not just the slash.

A prefix is a boundary in bits.

Classless Inter-Domain Routing writes an address followed by the number of leading network bits. In 192.0.2.17/27, 27 bits identify the network and five bits remain for addresses. Clearing those host bits produces 192.0.2.0; setting them produces 192.0.2.31. The same operation scales to all 128 IPv6 bits.

“Usable” depends on how the range is used.

The familiar host convention reserves the all-zero network address and all-one broadcast address. That makes a typical subnet’s conventional usable count two smaller than its total. Two important exceptions are shown explicitly: a /32 identifies one address, while RFC 3021 permits both addresses in a /31 on a point-to-point link. This calculator never subtracts those endpoints from the exact total.

No broadcast, much more address space.

IPv6 uses multicast instead of broadcast, so its first and last values are range boundaries—not reserved broadcast endpoints. Compressed output follows the familiar canonical rules: lowercase hexadecimal, suppressed leading zeroes, and one compressed longest run of zero groups. Expanded values remain available when fixed-width evidence is easier to audit.

Use ranges to investigate ownership, not to hide causality.

CIDR ranges appear in provider allowlists, firewall rules, routing announcements, reputation datasets, and log searches. For email, a range can help group adjacent sending IPs or verify that a callback source falls inside a published provider block. Deliverability decisions still belong at the narrowest observable level: a poor result on one IP does not prove that every address in a broad allocation behaves the same way.

CIDR calculator FAQ.