CIDR Calculator
Type an address with a prefix. Results update as you type.
Binary
Common IPv4 prefixes
| CIDR | Subnet mask | Hosts |
|---|---|---|
| /8 | 255.0.0.0 | 16,777,214 |
| /12 | 255.240.0.0 | 1,048,574 |
| /16 | 255.255.0.0 | 65,534 |
| /20 | 255.255.240.0 | 4,094 |
| /22 | 255.255.252.0 | 1,022 |
| /23 | 255.255.254.0 | 510 |
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /28 | 255.255.255.240 | 14 |
| /29 | 255.255.255.248 | 6 |
| /30 | 255.255.255.252 | 2 |
| /31 | 255.255.255.254 | 2 |
| /32 | 255.255.255.255 | 1 |
CIDR (Classless Inter-Domain Routing) notation writes a network as an address followed by a prefix length: 192.168.1.0/24 means “the first 24 bits identify the network”. The remaining bits number the hosts inside it.
The calculator masks off the host bits to find the network address, sets them all to one to find the broadcast address (IPv4 only), and counts the addresses in between. A dotted mask such as 255.255.255.0 is accepted too — it is simply another way of writing /24.
On IPv4, the first and last address of most subnets are reserved, so a /24 has 256 addresses but 254 usable hosts. Two exceptions: a /31 is used for point-to-point links where both addresses are usable (RFC 3021), and a /32 describes a single host.
Frequently asked questions
How many hosts are in a /24?
A /24 contains 256 addresses. On a normal IPv4 subnet the network and broadcast addresses are reserved, leaving 254 usable host addresses.
What is a wildcard mask?
The wildcard mask is the inverse of the subnet mask: host bits are 1 and network bits are 0. For a /24 it is 0.0.0.255. Cisco ACLs and OSPF configuration use it.
Does IPv6 have a broadcast address?
No. IPv6 replaces broadcast with multicast, so every address in an IPv6 prefix can be assigned. The all-zeros host address is the Subnet-Router anycast address.
Why does my result say host bits were set?
You entered an address that is not the first address of the network, such as 10.1.2.3/8. The calculator shows the network it belongs to (10.0.0.0/8).