THE FIELD NOTES / ADDRESSING
CIDR, without the guesswork.
The number after the slash tells you how much of an address belongs to the network.
One slash. A boundary.
In 192.168.10.42/24, the first 24 of the 32 bits identify the network. The remaining 8 bits cover 256 addresses. The canonical network is 192.168.10.0/24.
Address count is not always host count
For ordinary IPv4 subnets through /30, the network and broadcast addresses are excluded from usable host counts. A /31 point-to-point link uses both addresses; /32 identifies one address. IPv6 has no broadcast address and does not use that same subtract-two rule. Address assignment and subnet-router anycast still matter.
The same idea applies to IPv6
IPv6 has 128 address bits. A /64 therefore contains 2⁶⁴ addresses. That is why this toolbox shows exact counts rather than rounding them to an imprecise JavaScript number.
A subnet is not a firewall rule
Belonging to a range does not mean an address is reachable, authorized or assigned to a device. Routing, address allocation and security policy answer different questions.