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CIDR expresses network size using a prefix. It helps calculate ranges, segment networks and write firewall rules.
In IPv4, /24 usually represents 256 total addresses.
The mask separates the network part from the host part.
CIDR is common in firewalls, allowlists and routes.
CIDR lets you say “this whole block” without listing every IP. It is essential for support, internal networks and perimeter security.
A rule that is too broad can allow more sources than needed; a rule that is too narrow can block legitimate users.
In IPv6, prefixes work the same conceptually, but sizes are much larger. A /64 is often a complete local network.
FAQ
In IPv4 it usually represents a single IP. In IPv6 a single address is usually /128.
Not directly. CIDR defines ranges; countries use geolocation databases that can change.