CIDR is the mechanism for supporting hierarchical routing in the Internet backbone. Subnetting moves the network/host division line further rightwards; CIDR allows moving it to the left as well. With subnetting, the revised division line is visible only within the organization that owns the IP network address; subnetting is not visible outside. CIDR allows aggregation of IP address blocks in a way that is visible to the Internet backbone.
When CIDR was introduced in 1993, the following were some of the justifications for it, all relating to the increasing size of the backbone IP forwarding tables, and expressed in terms of the then-current Class A/B/C mechanism:
The Internet is running out of Class B addresses (this happened in the mid-1990’s)
There are too many Class C’s (the most numerous) for backbone forwarding tables to be efficient
Eventually IANA (the Internet Assigned Numbers Authority) will run out of IP addresses (this happened in 2011)
Assigning non-CIDRed multiple Class C’s in lieu of a single Class B would have helped with the first point in the list above, but made the second point worse.
Ironically, the current (2013) very tight market for IP address blocks is likely to lead to larger and larger backbone IP forwarding tables, as sites are forced to use multiple small address blocks instead of one large block.
By the year 2000, CIDR had essentially eliminated the Class A/B/C mechanism from the backbone Internet, and had more-or-less completely changed how backbone routing worked. You purchased an address block from a provider or some other IP address allocator, and it could be whatever size you needed, from /32 to /15.
What CIDR enabled is IP routing based on an address prefix of any length; the Class A/B/C mechanism of course used fixed prefix lengths of 8, 16 and 24 bits. Furthermore, CIDR allows different routers, at different levels of the backbone, to route on prefixes of different lengths. If organization P were allocated a /10 block, for example, then P could suballocate into /20 blocks. At the top level, routing to P would likely be based on the first 10 bits, while routing within P would be based on the first 20 bits.
CIDR was formally introduced by RFC 1518and RFC 1519 . For a while there were strategies in place to support compatibility with non-CIDR-aware routers; these are now obsolete. In particular, it is no longer appropriate for large-scale routers to fall back on the Class A/B/C mechanism in the absence of CIDR information; if the latter is missing, the routing should fail.
One way to look at the basic strategy of CIDR is as a mechanism to consolidate multiple network blocks going to the same destination into a single entry. Suppose a router has four class C’s all to the same destination:
184.108.40.206/24 ⟶ foo220.127.116.11/24 ⟶ foo18.104.22.168/24 ⟶ foo22.214.171.124/24 ⟶ foo
The router can replace all these with the single entry
126.96.36.199/22 ⟶ foo
It does not matter here if foo represents a single ultimate destination or if it represents four sites that just happen to be routed to the same next_hop.
It is worth looking closely at the arithmetic to see why the single entry uses /22. This means that the first 22 bits must match 188.8.131.52; this is all of the first and second bytes and the first six bits of the third byte. Let us look at the third byte of the network addresses above in binary:
200.7.000000 00.0/24 ⟶ foo200.7.000000 01.0/24 ⟶ foo200.7.000000 10.0/24 ⟶ foo200.7.000000 11.0/24 ⟶ foo
The /24 means that the network addresses stop at the end of the third byte. The four entries above cover every possible combination of the last two bits of the third byte; for an address to match one of the entries above it suffices to begin 200.7 and then to have 0-bits as the first six bits of the third byte. This is another way of saying the address must match 184.108.40.206/22.
Most implementations actually use a bitmask, eg 255.255.252.0, rather than the number 22. Note 252 is, in binary, 1111 1100, with 6 leading 1-bits, so 255.255.252.0 has 8+8+6=22 1-bits followed by 10 0-bits.
The IP delivery algorithm of 7.5 The Classless IP Delivery Algorithm still works with CIDR, with the understanding that the router’s forwarding table can now have a network-prefix length associated with any entry. Given a destination D, we search the forwarding table for network-prefix destinations B/k until we find a match; that is, equality of the first k bits. In terms of masks, given a destination D and a list of table entries ⟨prefix,mask⟩ = ⟨B[i],M[i]⟩, we search for i such that (D & M[i]) = B[i].
But what about the possibility of multiple matches? For subnets, avoiding this was the responsibility of the subnetting site, but responsibility for avoiding this with CIDR is much too distributed to be declared illegal by IETF mandate. Instead, CIDR introduced the longest-match rule: if destination D matches both B1/k1 and B2/k2, with k1 < k2, then the longer match B2/k2 match is to be used. (Note that if D matches two distinct entries B1/k1 and B2/k2 then either k1 < k2 or k2 < k1).