Type an address like 192.168.1.10/24, or an address and pick a subnet mask. Everything is worked out in your browser, and nothing is sent anywhere.
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An IPv4 address is 32 bits, written as four numbers from 0 to 255. The prefix (the number after the slash) says how many of those bits identify the network; the rest identify hosts on it. A /24 leaves 8 host bits, so 2⁸ = 256 addresses.
The network address is the IP with all host bits set to 0, and the broadcast address has them all set to 1. Neither can be given to a device, so a normal subnet has 2host bits − 2 usable addresses: 254 for a /24, 62 for a /26, 2 for a /30.
The subnet mask is the prefix written as an address (/24 = 255.255.255.0), and the wildcard mask is its inverse (0.0.0.255), as used in router access lists. Splitting a network adds prefix bits: a /24 splits into two /25s, four /26s, and so on.
256 addresses, of which 254 are usable: the first (network) and last (broadcast) are reserved. A /23 has 510 usable, a /25 has 126, and a /26 has 62.
Classless Inter-Domain Routing notation writes an address with its prefix length, like 10.0.0.0/8. It replaced the old fixed class A, B and C networks so networks can be any size.
Count the 1 bits in the mask: 255.255.255.0 is 24 ones (/24), 255.255.255.192 is 26 (/26), 255.255.0.0 is 16 (/16). The prefix menu shows both forms.
10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 (RFC 1918) are for private networks and aren’t routed on the internet. 100.64.0.0/10 is for carrier-grade NAT, and 169.254.0.0/16 is link-local (self-assigned).
The inverse of the subnet mask: 0 bits where the mask has 1s. It’s used in Cisco access lists and OSPF; the wildcard for a /24 is 0.0.0.255.
