CIDR stands for "Classless Inter-Domain Routing". But what's a domain in this case? Seems to not be the same kind of domain as DNS deals with...

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It is a routing domain. That is a block of addressing controlled by a single entity. For example, Company X could be assigned, Company Y could be assigned, and Company Z could be assigned

In the deprecated classful routing system, that could not happen because the whole block would need to be assigned to a single entity (domain). Within that entity, the network could be subnetted, but an entity could only advertise the classful network. With classless routing, multiple entities can advertise small blocks of the formerly classful addressing.

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