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About this tool
Convert between CIDR notation, start-end IP ranges and Cisco wildcard masks in bulk, with minimal CIDR splitting for arbitrary ranges.
The CIDR to IP Range Converter translates IPv4 networks between their three common notations — CIDR blocks (RFC 4632), start–end address ranges and Cisco ACL wildcard masks — in bulk, one entry per line. Arbitrary ranges that do not align to a power-of-two boundary are decomposed into the minimal list of CIDR blocks using the standard greedy alignment algorithm, and non-contiguous wildcard masks are detected and rejected with an explanation. It is built for network and cloud engineers moving allowlists between firewalls, ACLs and cloud security groups that each demand a different format.
Open CIDR to IP Range Converter on AltFTool — it loads instantly in your browser.
Paste your networks into 'Entries to convert (one per line)'; the accepted formats printed under the box are 10.0.0.0/24, 10.0.0.1 - 10.0.0.99, a wildcard pair such as 172.16.5.0 0.0.0.255, and a bare address like 10.0.0.5.
Every line is converted as you type — unaligned ranges are decomposed by the greedy algorithm into the fewest CIDR blocks, so 10.0.0.5 - 10.0.0.16 comes back as a /32, /31, /29 and /32, and unparseable lines are listed underneath.
Read the table columns Input, 'Start – end range', 'CIDR block(s)', 'Wildcard entry' and Addresses — a wildcard is shown only where the range is exactly one CIDR — then press 'Copy results'.
Every line is shown as a range, as CIDR block(s) and — when exact — as a wildcard entry.
Unaligned ranges are decomposed into the fewest possible blocks, never one-per-address.
Non-contiguous wildcard masks are flagged instead of being silently misconverted.
If the range starts on a power-of-two boundary and its length is a power of two, it is a single CIDR: 10.0.0.0–10.0.0.255 is 10.0.0.0/24. Otherwise it must be split: the greedy algorithm repeatedly takes the largest block that starts at the current address and fits the remainder, so 10.0.0.5–10.0.0.16 becomes 10.0.0.5/32, 10.0.0.6/31, 10.0.0.8/29 and 10.0.0.16/32.
A wildcard mask is the bitwise inverse of the subnet mask: /24 has subnet mask 255.255.255.0 and wildcard 0.0.0.255. Wildcards are used in Cisco ACLs and OSPF network statements, where 0 bits mean 'must match' and 1 bits mean 'ignore'. Unlike subnet masks, wildcards may legally be non-contiguous, in which case they match a scattered set that no single CIDR can represent.
CIDR blocks are always a power-of-two in size and start at a multiple of that size. A range like 10.0.0.5–10.0.0.16 holds 12 addresses starting at an odd offset, so no single prefix covers exactly those addresses — it takes four blocks. This tool always emits the provably minimal set.
2^(32 − prefix): a /24 holds 256 addresses, a /16 holds 65,536 and a /8 holds 16,777,216. That count includes the network and broadcast addresses; usable host counts are two lower on ordinary subnets (see a subnet calculator for /31 and /32 special cases).
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