CIDR (Classless Inter-Domain Routing) notation compresses an IP range into an address and a prefix, like 192.168.1.0/24. This tool expands any IPv4 CIDR block into its full details: the network address, broadcast address, usable host range, subnet mask, and the total and usable address counts. It is built for network engineers planning subnets and anyone decoding a firewall or routing rule.
How it works
The prefix is the number of leading bits that form the network portion; the rest address hosts. The tool converts the IP to a 32-bit integer, builds a mask with the top prefix bits set, then derives:
- Network = IP AND mask (the lowest address in the block)
- Broadcast = network OR (NOT mask) (the highest address)
- Total addresses = 2^(32 − prefix)
- Usable hosts = total − 2, since the network and broadcast addresses are reserved
/31 (point-to-point links) and /32 (single host) are special cases with 2 and 1 usable addresses respectively.
Example
For 192.168.1.0/24:
| Field | Value |
|---|---|
| Network | 192.168.1.0 |
| Broadcast | 192.168.1.255 |
| Subnet mask | 255.255.255.0 |
| Usable range | 192.168.1.1 – 192.168.1.254 |
| Total / usable | 256 / 254 |
All calculations are bitwise integer maths performed locally in your browser.
Common prefix sizes and their host counts
Different tasks call for different block sizes. Here are the most frequently used prefixes:
| Prefix | Total addresses | Usable hosts | Typical use |
|---|---|---|---|
| /30 | 4 | 2 | Point-to-point router links |
| /29 | 8 | 6 | Very small office segments |
| /28 | 16 | 14 | Small VLAN or device cluster |
| /27 | 32 | 30 | Small departmental network |
| /24 | 256 | 254 | Standard office LAN segment |
| /22 | 1,024 | 1,022 | Medium campus or data-centre block |
| /16 | 65,536 | 65,534 | Large enterprise or cloud VPC |
When to use this tool
Subnet planning. When carving an address space into VLANs, confirm that each /prefix delivers enough usable addresses for the devices you expect to connect — and remember to include router interfaces, printers, and spare addresses, not just workstations.
Firewall rule audits. Security teams decode CIDR blocks in ACLs, AWS security groups, and cloud firewall rules to verify the scope of an allow or deny. Paste any block here to see exactly which addresses it covers before approving a rule change.
Routing table interpretation. BGP and OSPF routes are expressed as CIDR blocks. Expanding a route to its address range makes it easy to check for overlaps or unintended coverage before pushing a configuration change.
Edge cases to know
A /32 covers exactly one host — the network and broadcast fields are the same as the address itself — and is used to add a static host route or loopback entry. A /0 covers the entire IPv4 address space (all 4,294,967,296 addresses) and is the catch-all default route in routing tables. Between those extremes, always check that your block boundaries are correctly aligned: a network address must be a multiple of the block size, so 192.168.1.1/24 is not a valid network address (the network is 192.168.1.0/24), though many tools accept and silently correct the input.