Last updated: 2026-06-23

IP Address Range

An IP address range is a contiguous span of IPv4 addresses defined by a first and a last address. This calculator counts how many addresses the span contains and, when the span lines up with a single network block, reports the equivalent CIDR notation. Network administrators reach for it when sizing a DHCP scope, writing a firewall rule, or checking whether a range from a hosting provider can be summarized into one route.

From dotted-decimal to integers

Every IPv4 address is a 32-bit number written as four 8-bit octets. The address $a.b.c.d$ corresponds to the integer

$\text{int} = a \cdot 256^3 + b \cdot 256^2 + c \cdot 256 + d$

Once both endpoints are integers, the number of addresses in the range is simply the difference plus one, because the range includes both ends:

$N = \text{int}(\text{end}) - \text{int}(\text{start}) + 1$

The "plus one" is the part people most often forget. A range from .0 to .255 looks like 255 steps, but it holds 256 addresses.

When a range is a single CIDR block

CIDR notation describes a block by its starting address and a prefix length, such as 192.168.1.0/24. A block always contains a power-of-two number of addresses — a /24 holds $2^{8} = 256$, a /25 holds $2^{7} = 128$ — and it must begin on a boundary that is a multiple of its own size. A range can be written as one block only when both conditions hold: its length is a power of two, and its start address is aligned to that length. If either fails, the range still has a well-defined count, but covering it without gaps or overlap requires several blocks of differing sizes.

Worked example

Consider the range 192.168.1.0 through 192.168.1.127:

The count is 128, which is $2^{7}$, and the start address 192.168.1.0 is a multiple of 128, so the range collapses to a single block: 192.168.1.0/25. Shorten the range by one address — ending at 192.168.1.126 — and the count drops to 127, which is not a power of two, so no single CIDR block fits.

To break an arbitrary span into its constituent subnets, or to inspect the mask and host range of one block, see the Subnet (CIDR) Calculator. To understand how the underlying integers are formed, the Number Base Converter shows the binary and hexadecimal views of each octet.