IP Range Expander

Two modes in one tool. CIDR mode takes something like 192.168.1.0/24 and returns the network address, the broadcast address, the first and last usable host, the subnet mask, the wildcard mask, the total and usable address counts, and the expanded list of addresses up to the limit you set. Range mode takes a start and an end address and decomposes the span into the smallest set of CIDR blocks that covers it exactly — no more, no less — which is what firewall rules, ACLs and route tables want when you can only express things as prefixes.

The decomposition is worth understanding, because the answer is often larger than people expect. A range that starts or ends off a power-of-two boundary cannot be one prefix. 10.0.0.1 to 10.0.3.255 needs ten blocks — a /32, then a /31, /30, /29, /28, /27, /26, /25, /24 and finally a /23 — because each step can only double once the alignment allows it. If a rule set has a hard limit on entries, that expansion is the number to check before committing to a range.

The usable-host count follows the special cases rather than the naive formula. A /24 holds 256 addresses of which 254 are assignable, since the first is the network identifier and the last is the broadcast address. But a /31 has no room for either and RFC 3021 makes both of its addresses usable on a point-to-point link, and a /32 is a single host — so the tool reports 2 and 1 rather than 0 and −1, which is what subtracting two blindly would give.

Two limits to know. This is IPv4 only; IPv6 prefixes are not accepted, and expanding one would be meaningless anyway — a single /64 holds more addresses than IPv4 has in total. And the expansion is capped deliberately: a /16 is 65,536 addresses and a /8 is over sixteen million, so the list is truncated at your configured limit and marked as such rather than locking up the tab. Nothing here touches the network — no DNS lookups, no reachability tests — so a result says only what the arithmetic says, never whether an address answers.