| Class A and B addresses make up 75
percent of the IPv4 address space, but a relative handful of
organizations (fewer than 17,000) can be assigned a Class A or B
network number. Class C network addresses are far more numerous than
Class A and Class B addresses, although they account for only 12.5
percent of the possible 4 billion (232) IP hosts, as shown in the
Figure.
Unfortunately, Class C addresses are
limited to 254 hosts, which will not meet the needs of larger
organizations that can not acquire a Class A or B address. Even if
there were more Class A, B, and C addresses, too many network
addresses would cause Internet routers to grind to a halt under the
weight of enormous routing tables.
Ultimately, the classful system of IP
addressing, even with subnetting, could not scale to effectively
handle global demand for Internet connectivity. As early as 1992,
the Internet Engineering Task Force (IETF) identified two specific
concerns:
- Exhaustion of the remaining,
unassigned IPv4 network addresses. At the time, the Class B
space was on the verge of depletion.
- The rapid and substantial increase
in the size of the Internet's routing tables is because of its
growth. As more Class C's came online, the resulting flood of
new network information threatened Internet routers' capability
to cope effectively.
In the short term, the IETF decided
that a retooled IPv4 would have to hold out long enough for
engineers to design and deploy a completely new Internet Protocol.
That new protocol, IPv6, solves the address crisis by using a
128-bit address space. After years of planning and development, IPv6
promises to be ready for wide-scale implementation, although it
continues (for the most part) to wait in the wings.
One reason that IPv6 has not been
rushed into service is that the short-term extensions to IPv4 have
been so effective. By eliminating the rules of class, IPv4 now
enjoys renewed viability.
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