Networking also: Classless Inter-Domain Routing

CIDR

IP address + prefix length notation (e.g. 10.0.0.0/24).

In plain terms

Replaced the rigid Class A/B/C system in 1993. The /N tells you how many high bits identify the network.

Origin

RFC 1518 / 1519 (1993). Replaced the rigid Class A/B/C scheme that was rapidly exhausting the IPv4 space.

Where it shows up in production

VPC subnets Every cloud VPC asks for a CIDR block — /16 typical for a VPC, /24 for a subnet.
BGP route aggregation ISPs advertise /22 or /20 supernets instead of millions of /32s.

On Semicolony

Sources & further reading

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