How did cdir ease the problem of fast depleting ip4 addressing?

Answer 1

CIDR = Classless interdomain addressing

Before CIDR, the network portion of IP address were constrained to be 8, 16, or 24 bits in length, an addressing scheme known as *classful* addressing, since subnets with 8, 16, 24 bit subnet addresses were known as class A, B, C networks, respectively.

This cause problem for rapidly growing number of organisations with small and medium sized subnets. A class C (24 bit) subnet could accomodate only up to 2^8 - 2 = 254 (nodes/hosts) (2 of the 256 addresses are reserved for special use) which is too small for many organisations. However, Class B is too large (65634 hosts).

CIDR generalises the notion of subnet addressing. As with subnet addessing, the 32-bit (IPv4) address is divided into two parts and again has the dotted-decimal form a.b.c.d/x where x indicates the number of bits in the first part of the address. The x most significant bits of an address of the form a.b.c.d/x constitute the network portion of the IP address and are often referred to as the prefix (or network prefix) of the address.

An organisation is typically assigned a block of contiguous addresses, that is , a range of addresses with a common prefix. In this case, the IP addresses of devices within the organisation will share the common prefix. The remaining bits of an address can be thought of as distinguishing among the devices within the organisation, all of which have the same network prefix.

Reference: Computer Networking: A top down approach, James F. Kurose, Keith W. Ross