At the beginning, IANA gave requestors an
entire class A network space thereby granting requestors 16.7 million
addressesmany more than necessary. Realizing their error, they began to assign
class B networksagain, providing far too many addresses for the average
requestor. As the Internet grew, it quickly became clear that allocating class A
and class B networks to every requestor did not make sense. Even their later
action of assigning class C banks of addresses still squandered address space,
as most companies didn't require 254 IP addresses. Since IANA could not revoke
currently allocated address space, it became necessary to deal with the
remaining space in a way that made sense. One of these ways was through the use
of Classless Inter-Domain Routing CIDR
IPv4
space is becoming scarcer by the day. By 2005, some estimates place the number
of worldwide Internet users at over one billion. Given the fact that many of
those users will have a cellular phone, a home computer, and possibly a computer
at work, the available IP address space becomes critically tight. China has
recently requested IP addresses for each of their students, for a total of
nearly 300 million addresses. Requests such as these, which cannot be filled,
demonstrate this shortage. When IANA initially began allotting address space,
the Internet was a small and little- known research network. There was very
little demand for addresses and class A address space was freely allocated.
However, as the size and importance of the Internet started to grow, the number
of available addresses diminished, making obtaining a new IP difficult and much
more expensive. NAT and CIDR are two separate responses to this scarcity. NAT is
an individual solution allowing one site to funnel its users through a single IP
address. CIDR allows for a more efficient division of network address block.
Both solutions, however, have limitations.
.
CIDR allows network blocks to be allocated
outside of the well-defined class A/B/C ranges. In an effort to get more mileage
from existing class C network blocks, CIDR allows administrators to divide their
address space into smaller units, which can then be allocated as individual
networks. This made it easier to give IPs to more people because space could be
allocated by need, rather than by predefined size-of-space. For example, a
provider with a class C subnet could choose to divide this network into 32
individual networks, and would use the network addresses and subnet masks to
delineate the boundaries. A sample CIDR notation looks like this:
10.10.0.64/29
In this example, the /29 denotes the subnet
mask, which means that the first 29 bits of the address are the subnet. It could
also be noted as 255.255.255.248, which gives
this network a total of six usable addresses.
While CIDR does deal with the problem in a quick and easy way,
it doesn't actually create more IP addresses, and it does have some additional
disadvantages. First, its efficiency is compromised since each allocated network
requires a broadcast IP and a network address IP. So if a provider breaks a
class C block into 32 separate networks, a total of 64 individual IPs are wasted
on network and broadcast IPs. Second, complicated CIDR networks are more prone
to configuration errors. A router with an improper subnet mask can cause an
outage for small networks it serves.