Firewall rules, router ACLs, and spreadsheet planning all speak in CIDR — but humans think in first-and-last addresses. The Gizmo 421 IP Range Expander translates both directions for IPv4 and IPv6: give it a CIDR and get the full range, or give it a range and get the minimal set of CIDR blocks that covers it.
Two directions, one input
Type 192.168.1.0/24 and you get network address, broadcast, usable host range and count, subnet mask, and wildcard mask. Type 10.0.0.10 - 10.0.0.99 and you get the range’s address count plus the minimal CIDR list covering it — the same thing every firewall GUI quietly computes for you. IPv6 works in both directions too, using exact 128-bit math, with addresses displayed in proper RFC 5952 compressed form.
Why “minimal CIDRs” matters
DNS reverse zones, router summaries, and AWS security group rules all accept CIDR blocks, not arbitrary ranges — and an unaligned range can never be one block. The tool returns the smallest set that tiles your range exactly (a 90-address range becomes 6 blocks), which is directly pasteable into ACLs and route summarizations. For IPv4, hosts on /31 and /32 networks count as usable per RFC 3021, matching modern point-to-point practice.
Common questions
Can it mix IPv4 and IPv6 in one range? No — a range must be one family on both ends. Why does my range come back as multiple CIDRs? Only aligned ranges collapse to a single block; the tool shows the minimum tile set. Are the huge IPv6 counts real? Yes — a /48 really does contain 2^80 addresses; the tool computes them exactly with BigInt, not with floating point.