How CIDR Works

CIDR implementations require certain support be embedded within the network routing protocols. When first implemented on the Internet, the core routing protocols like BGP (Border Gateway Protocol) and OSPF (Open Shortest Path First) were updated to support CIDR. Obsolete or less popular routing protocols may not support CIDR.

CIDR aggregation requires the network segments involved to be contiguous (numerically adjacent) in the address space. CIDR cannot, for example, aggregate 192.168.12.0 and 192.168.15.0 into a single route unless the intermediate .13 and .14 address ranges are included (i.e., the 192.168.12/22 network).

Internet WAN or backbone routers (those that manage traffic between Internet Service Providers) all generally support CIDR to achieve the goal of conserving IP address space. Mainstream consumer routers often do not support CIDR, therefore private networks (including home networks) and even small public networks (LANs) often do not employ it.