Why does a /24 have 254 usable hosts and not 256?

The first address identifies the network itself and the last is the broadcast address, so neither can be assigned to a host. That is why the usable count is two fewer than the total, for every prefix up to /30.

What is a wildcard mask for?

It is the subnet mask inverted, and Cisco access control lists are written with it. A /24 has the mask 255.255.255.0 and the wildcard 0.0.0.255 — where a mask bit means "must match", a wildcard bit means "ignore".

Which ranges are private?

10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, defined by RFC 1918 and not routable on the public internet. The scope row labels these, along with loopback, link-local, carrier-grade NAT and multicast ranges.

Does this handle IPv6?

Not yet — IPv4 only. IPv6 subnetting is arithmetically simpler in some ways but the addresses need 128-bit handling and a different notation, so it belongs in its own tool.

CIDR Calculator

cidr → range

Work out the network, broadcast and usable host range of a subnet.

IPv4 block

An address on its own is treated as a single host, the same as /32.

try

network

192.168.1.0/24

subnet mask

255.255.255.0

wildcard

for ACLs written the Cisco way

0.0.0.255

usable hosts

192.168.1.1 – 192.168.1.254

broadcast

192.168.1.255

host count

254 usable of 256

scope

Private (RFC 1918)

Is an address inside this block?

split into /26 subnets

192.168.1.0/26192.168.1.64/26192.168.1.128/26192.168.1.192/26

About CIDR Calculator

Enter an IPv4 block in CIDR notation and get its network and broadcast addresses, the usable host range, the subnet mask, the wildcard mask, and how many addresses it actually holds. An address without a prefix is treated as a single host.

Two edge cases are handled properly rather than reported as ordinary blocks. A /32 is one address with no range at all. A /31 has no broadcast address: RFC 3021 makes both of its addresses usable, which is precisely why point-to-point links use it — and a calculator that subtracts two for network and broadcast will tell you it has zero usable hosts, which is wrong.

There is also a membership check, for the question people usually arrive with — whether a particular address falls inside a block — and a split view showing the subnets one level down.

Common questions

Why does a /24 have 254 usable hosts and not 256?: The first address identifies the network itself and the last is the broadcast address, so neither can be assigned to a host. That is why the usable count is two fewer than the total, for every prefix up to /30.
What is a wildcard mask for?: It is the subnet mask inverted, and Cisco access control lists are written with it. A /24 has the mask 255.255.255.0 and the wildcard 0.0.0.255 — where a mask bit means "must match", a wildcard bit means "ignore".
Which ranges are private?: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, defined by RFC 1918 and not routable on the public internet. The scope row labels these, along with loopback, link-local, carrier-grade NAT and multicast ranges.
Does this handle IPv6?: Not yet — IPv4 only. IPv6 subnetting is arithmetically simpler in some ways but the addresses need 128-bit handling and a different notation, so it belongs in its own tool.

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