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Appendix A. CIDR and CIDR Notation
Classless Inter-Domain Routing (CIDR) is a strategy for IP address assignment originally specified in 1993
that had the aims of "conserving the address space and limiting the growth rate of global routing state". The
current specification for CIDR is in RFC4632 [http://tools.ietf.org/html/rfc4632].
The pre-CIDR era
CIDR replaced the original class-based organisation of the IP address space, which had become wasteful of
address space, and did not permit aggregation of routing information.
In the original scheme, only three sizes of network were possible :
• Class A : 128 possible networks each with 16,777,216 addresses
• Class B : 16384 possible networks each with 65,536 addresses
• Class C : 2097152 possible networks each with 256 addresses
Every network, including any of the large number of possible Class C networks, required an entry in global
routing tables - those used by core Internet routers - since it was not possible to aggregate entries that had the
same routing information. The inability to aggregate routes meant global routing table size was growing fast,
which meant performance issues at core routers.
The position and size of the network ID and host ID bitfields were implied by the bit pattern of some of the
most significant address bits, which segmented the 32-bit IPv4 address space into three main blocks, one for
each class of network.
CIDR
The prefix notation introduced by CIDR was, in the simplest sense, "to make explicit which bits in a 32-bit
IPv4 address are interpreted as the network number (or prefix) associated with a site and which are the used
to number individual end systems within the site". In this sense, the 'prefix' is the N most significant bits that
comprise the network ID bitfield.
CIDR notation is written as :-
IPv4 : Traditional IPv4 'dotted-quad' number, followed by the "/" (slash) character, followed by a decimal
prefix-length value between 0 and 32 (inclusive)
IPv6 : IPv6 address, followed by the "/" (slash) character, followed by a decimal prefix-length value between
0 and 128 (inclusive)
Where formerly only three network sizes were available, CIDR prefixes may be defined to describe any power
of two-sized block of between one and 2^32 end system addresses, that begins at an address that is a multiple
of the block size. This provides for far less wasteful allocation of IP address space. The size of the range is
given by 2^M, where M = 32 - prefix_length
Routing destinations
As well as being used to define a network (subnet), the CIDR notation is used to define the destination in
a routing table entry, which may encompass multiple networks (with longer prefixes) that are reachable by
using the associated routing information. This, therefore, provides the ability to create aggregated routing table
entries.
For example, a routing table entry with a destination of 10.1.2.0/23 specifies the address range 10.1.2.0
to 10.1.3.255 inclusive. As an example, it might be that in practice two /24 subnets are reachable via this