What does it do?
It describes a CIDR block: netmask, wildcard, network and broadcast addresses, the range of usable hosts and how many there are. It also tells you whether an address is private, public, loopback or CGNAT, checks whether an IP belongs to the block and splits the block into smaller subnets.
How to use it
- Type a block such as
10.0.0.0/16, an IP with its mask (10.0.0.1 255.255.0.0) or a single IP.
- Type an IP in the second field to check if it is inside the range: useful for firewall rules, security groups and allowlists.
- Choose a prefix in Split into to plan subnets, for example a
/16 VPC split into /24 subnets.
Cheat sheet
| Prefix | Netmask | Addresses |
|---|
| /32 | 255.255.255.255 | 1 |
| /30 | 255.255.255.252 | 4 |
| /28 | 255.255.255.240 | 16 |
| /24 | 255.255.255.0 | 256 |
| /20 | 255.255.240.0 | 4,096 |
| /16 | 255.255.0.0 | 65,536 |
| /8 | 255.0.0.0 | 16,777,216 |
Frequently asked questions
›Why are there 254 usable hosts in a /24 and not 256?
In IPv4 the first address of a block identifies the network and the last one is the broadcast address, so neither can be given to a host. The exceptions are /31 (point-to-point links, RFC 3021) and /32 (a single host).
›What is the wildcard mask?
The inverse of the netmask: 0.0.0.255 for a /24. Cisco ACLs and OSPF configurations use it instead of the netmask.
›Does it work with IPv6?
Yes. IPv6 has no broadcast, so every address in the block counts. Sizes are computed exactly, even for a /64 with 18 trillion trillion addresses.