CIDR Range Calculator

What the CIDR range calculator returns

A CIDR value such as 10.20.30.0/24 describes a contiguous IPv4 block. This calculator turns that notation into practical boundaries: normalized network address, first and last address under the displayed semantics, broadcast value where applicable, and the total number of addresses in the prefix.

You can enter a host address instead of the canonical network address. For example, 172.16.40.23/27 is normalized before the range is calculated. If you want the underlying theory, review the IP addressing guide and the subnetting guide.

How to use the CIDR range calculator

Enter an IPv4 address followed by a slash and prefix length, then press Calculate range. The tool validates the prefix, finds the real network boundary and calculates the end of the block. You do not need to work out the subnet increment manually first.

The result is useful when documenting networks, validating DHCP scopes, reviewing firewall objects, checking static routes, or confirming the boundaries of a VLAN. If you only need a membership test for one IP, use the IP-in-subnet checker.

Example: 172.16.40.23/27

A /27 contains 32 addresses. In the fourth octet, /27 blocks start at 0, 32, 64, 96 and so on. The address 172.16.40.23 therefore belongs to the block beginning at 172.16.40.0 and ending at 172.16.40.31.

Under traditional IPv4 LAN semantics, the first usable host is 172.16.40.1, the last usable host is 172.16.40.30, and 172.16.40.31 is the broadcast address. The total block size remains 32 addresses. Compare this with the interactive CIDR table when learning common prefixes.

How prefix length determines the block size

IPv4 has 32 bits. A /24 leaves 8 host bits and therefore contains 2^8, or 256, addresses. A /25 contains 128, a /26 contains 64, a /27 contains 32, and each additional network bit halves the block again. This relationship is why the prefix alone is enough to calculate the range.

The dotted subnet mask expresses the same boundary in another notation. If you receive a mask such as 255.255.255.224, convert it with the mask ↔ CIDR converter and then calculate the range.

Important /31 and /32 behavior

A /31 contains two addresses and is commonly used for IPv4 point-to-point links under RFC 3021. A /32 contains one address and is typically treated as a host route. These prefixes do not have a separate broadcast address in the same way as a traditional multi-host LAN.

The tool therefore treats their boundaries explicitly rather than applying a universal “network + broadcast are unusable” shortcut. If you are designing point-to-point links, use semantics appropriate to the technology instead of assuming every prefix behaves like a classic LAN.

Operational uses for exact CIDR boundaries

Knowing the exact range helps prevent subtle configuration errors. A firewall object that starts one address too early may include a neighboring subnet. A DHCP pool that exceeds the broadcast boundary may overlap another network. A route written with the wrong prefix can attract traffic that should follow a different path.

For multiple prefixes, the subnet overlap checker can detect containment and collisions. For route summarization, the CIDR aggregator finds exact combinations that can be merged.

Frequently asked questions

Next step

Use this calculator to understand one prefix, then move to overlap detection, range-to-CIDR conversion or aggregation when you need to validate and restructure larger address plans.