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13.1. The IPv4 Problem and Patchwork Solutions
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).
13.1.1. CIDR
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.
13.1.2. NAT
Network
Address Translation (NAT) provides some relief for the IP
address space dilemma, and without it, we'd
currently be well out of usable IP space. NAT provides a many-to-one
translation, meaning that many machines can share the same IP
address. This also provides some privacy and security for the
machines behind the NAT device, since individually identifying them
is more difficult. There are also some disadvantages to
NATprimarily that some older protocols aren't
designed to handle redirection.
