CIDR replaced the older "classful" addressing (Class A/B/C blocks) in 1993. A CIDR block is written as "address/prefix-length"; the prefix specifies how many leading bits are the network portion.
"/24" means the first 24 bits are network, leaving 8 bits (256 addresses) for hosts. "/32" is a single host. "/0" is the entire internet.
IPv6 uses the same notation but the math is bigger. "/64" — the typical home or office subnet — has 18 quintillion addresses.
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See also
- IPv4The original 32-bit IP addressing scheme — addresses look like 203.0.113.42 and there are ~4.3 billion of them.
- IPv6The 128-bit successor to IPv4 — addresses look like 2001:db8::1 and there are 2^128 of them.
- RFC 1918The IPv4 ranges set aside for private use — 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Not routed on the public internet.