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CIDR Calculator

Paste a CIDR block, including one written with a host address inside it. The calculator aligns that address down to the network, then lists the inclusive range and the usable hosts. Use the subnet calculator when you already have the address and prefix in separate fields.

CIDR prefix

65,534 usable hosts

00001010000000010000001000000011
/16
/8/16/24/32
Network bitsHost bits

Drag the line, or use the arrow keys. The result updates as it moves.

10.1.2.3/16 normalizes to 10.1.0.0/16.

10.1.0.0/16

Private network (RFC 1918). The whole block shares this label.

Network address

10.1.0.0

Broadcast address

10.1.255.255

Subnet mask

255.255.0.0

Total addresses

65,536

Usable hosts

65,534

First host

10.1.0.1

Last host

10.1.255.254

Wildcard mask

0.0.255.255

Historical class

A

On a conventional LAN the network and broadcast addresses are not hosts. Usable hosts are the addresses between them.

Historical class A describes the old classful split. Size the network with the CIDR prefix.

Binary

IP address
00001010.00000001.00000010.00000011
Subnet mask
11111111.11111111.00000000.00000000
Network address
00001010.00000001.00000000.00000000

Network and host bits

16 network bits16 host bits

network = address AND 255.255.0.0 = 10.1.0.0

broadcast = network OR 0.0.255.255 = 10.1.255.255

total = 2^16 = 65,536

usable = total - 2 = 65,534

Continue with this block in the range calculator, the wildcard mask, or the membership check.

What normalization changes

10.1.2.3/16 normalizes to 10.1.0.0/16. The first 16 bits stay (10.1). The host octets clear, so the block runs from 10.1.0.0 through 10.1.255.255 (65,536 addresses). Usable hosts are 10.1.0.1 through 10.1.255.254. Routing and ACL network fields want that cleared form. A noncontiguous mask cannot be written as a prefix, so this page rejects it.

Questions

No. 10.1.2.3/16 covers 10.1.0.0 through 10.1.255.255. The host octets do not shrink the prefix, and they do not change the network octets either.

It is in the result, derived from the prefix. The mask calculator is the shorter tool if you only need the dotted decimal conversion.

Sources

Reviewed 8 October 2026. The methodology page records what these labels do not cover.

Related tools

All toolsHow the numbers are derived