SeaMeWe4 Net Tools Subnet calculator

Subnet Calculator
for IPv4 networks

A subnet calculator splits an IPv4 network into smaller address ranges by applying a subnet mask or CIDR prefix, instantly returning the network address, broadcast address, usable host range and total host count for that range — no sign-up, no install, results update as you type.

Open the calculator

IPv4 subnet calculator

Enter an IP address and drag the slider — or type a dotted-decimal mask — to see every value for that subnet.

/0 /32 /24
Network / host bits

Network bits (dark)Host bits (light)

Network address192.168.1.0
Broadcast address192.168.1.255
First usable host192.168.1.1
Last usable host192.168.1.254
Usable hosts254
Wildcard mask0.0.0.255
Mask in binary11111111.11111111.11111111.00000000
IP classC
Address typePrivate (RFC 1918)
RFC 791 · RFC 4632Checked 25 Oct 2026

Subnet mask table

The most commonly used prefixes, their dotted-decimal mask, and how many addresses each one holds.

CIDR prefixSubnet maskTotal addressesUsable hosts
/8255.0.0.016,777,21616,777,214
/16255.255.0.065,53665,534
/20255.255.240.04,0964,094
/22255.255.252.01,0241,022
/24255.255.255.0256254
/25255.255.255.128128126
/26255.255.255.1926462
/27255.255.255.2243230
/28255.255.255.2401614
/29255.255.255.24886
/30255.255.255.25242
/31255.255.255.25422
/32255.255.255.25511

How the calculator works

Diagram of 192.168.1.0/24 showing 24 fixed network bits and 8 variable host bits across the four octets

The calculator reads the prefix length (or the dotted-decimal mask you type, which it converts to the same prefix) and uses it to split the 32-bit IPv4 address into a fixed network portion and a variable host portion. It then applies a bitwise AND between the address and the mask to find the network address, flips the host bits to find the broadcast address, and everything in between is the usable host range — the same arithmetic described in RFC 4632.

More networking tools

The calculator above covers IPv4 subnetting. These related tools are being added to SeaMeWe4 Net Tools next.

IP subnet calculatorA dedicated deep-dive version of the tool above, with more worked examples.CIDR calculatorConvert between CIDR notation, dotted masks and address ranges.IPv6 subnet calculatorThe same network/host split, for 128-bit IPv6 addressing.VLSM calculatorPlan several differently sized subnets out of one address block.AWS/Azure VPC subnet calculatorAccount for each cloud provider's reserved addresses per subnet.Fiber latency calculatorEstimate best-case round-trip time between two cities.

Frequently asked questions

What does a subnet mask actually do?

A subnet mask marks which bits of an IPv4 address identify the network and which bits identify the host. Bits set to 1 (e.g. 255.255.255.0) are the network portion; the remaining bits are available for host addresses inside that subnet.

How many usable hosts does a /24 have?

A /24 has 256 total addresses, but the first (network) and last (broadcast) address in the range can't be assigned to a device, so a standard /24 gives 254 usable host addresses. The same -2 rule applies to every prefix except /31 and /32, which have special uses.

Why do we subtract 2 host addresses?

The lowest address in any subnet identifies the network itself, and the highest address is reserved as the broadcast address for that subnet — neither can be assigned to an individual host. That leaves total addresses minus 2 as usable.

What is the difference between a subnet mask and CIDR notation?

They describe the same split in two formats. A subnet mask is written as four decimal numbers (255.255.255.0); CIDR notation writes the prefix length directly after the IP address as a slash and a number (/24). Both represent 24 network bits in this example.

Fiber latency estimator

Light in optical fiber travels at roughly two-thirds of its speed in a vacuum, and cables never run in a straight line. Adjust the route factor to see how detours add delay.

—ms round trip (fiber, best case)
Great-circle distance
—
Estimated cable length
—
One-way in fiber
—
Round trip in a vacuum
—

Numbers behind the latency estimate

Why real-world ping is always higher than the physics floor.

FactorTypical valueEffect on latency
Speed of light in fiber≈ 204,000 km/s (refractive index ≈ 1.47)About 4.9 µs per km, one way.
Route factor1.2–1.6× great-circle distanceCables follow coastlines, avoid hazards and stop at landing stations.
Terrestrial backhaulTens to hundreds of kmData still has to reach the landing station and the data center inland.
Routing and equipmentA few msEach router, switch and amplifier adds a little processing delay.
Congestion0 to 100+ msBusy links queue packets; this is where “lag” spikes usually come from.

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