A CIDR prefix such as 192.0.2.64/26 describes all IPv4 addresses whose first 26 bits match the prefix.

A host determines the network prefix by keeping the prefix bits and ignoring the remaining host bits. A /26 leaves $32-26=6$ host bits, so each such block contains $2^6=64$ IPv4 addresses.

Because block boundaries occur every 64 addresses, 192.0.2.64/26 spans 192.0.2.64 through 192.0.2.127. Thus 192.0.2.100 belongs to this prefix while 192.0.2.130 does not.

A host uses its configured prefix to decide whether a destination is on-link. Local destinations can be reached through the local link; nonlocal destinations are normally sent to a router.

Longer prefixes represent smaller, more specific networks. CIDR therefore supports both subnetting an address range and aggregating routes at larger scales.