Subnetting & IP Addressing

Smaller broadcast domains — every device in a subnet hears every broadcast; splitting a /16 into /24s means noisy traffic in one office doesn't flood every office.
Route summarization — a router can advertise one summarized route for an entire subnet instead of one route per host, keeping routing tables small.
Security boundaries — subnets are a natural place to enforce firewall rules, e.g. between a public-facing subnet and an internal database subnet.
Efficient allocation via VLSM — give each subnet only as many addresses as it actually needs.

IP address

CIDR prefix

/ 24

Network bits (fixed for every device on this subnet) Host bits (unique per device)

Defined by RFC 1918 and others. You'll run into these constantly in the calculator above.

Range	CIDR	Typical use
10.0.0.0 – 10.255.255.255	10.0.0.0/8	Large private networks (common in enterprises and clouds)
172.16.0.0 – 172.31.255.255	172.16.0.0/12	Private networks (Docker's default bridge network)
192.168.0.0 – 192.168.255.255	192.168.0.0/16	Private networks (most home routers)
127.0.0.0 – 127.255.255.255	127.0.0.0/8	Loopback (127.0.0.1 = localhost)
169.254.0.0 – 169.254.255.255	169.254.0.0/16	Link-local (APIPA), self-assigned when DHCP fails
224.0.0.0 – 239.255.255.255	224.0.0.0/4	Multicast (one-to-many delivery)

The network and broadcast addresses aren't usable hosts. That's why usable hosts = total addresses − 2 for anything /30 or larger.
/31 and /32 are special-cased by design. RFC 3021 lets a /31 use both of its addresses as real hosts on point-to-point links (e.g. router-to-router), and a /32 identifies a single host route with no subnet at all.
Classful addressing (Class A/B/C) is legacy. It hasn't determined real-world subnet sizes since the early 1990s — everything today is classless (CIDR), but the "Class" badge above is still useful shorthand for how big a block originally was.
A bigger prefix number means a smaller subnet. This trips up almost everyone at first: /24 (256 addresses) is much bigger than /30 (4 addresses) — the prefix counts fixed network bits, not host capacity.