TCP/IP Tutorial and Technical Overview - IBM Redbooks

Anycasting
Sometimes, the same IP services are provided by different hosts. For example, a
user wants to download a file using FTP and the file is available on multiple FTP
servers. Hosts that implement the same service provide an anycast address to
other hosts that require the service. Connections are made to the first host in the
anycast address group to respond. This process is used to guarantee the service
is provided by the host with the best connection to the receiver.
The anycast service is included in IPV6 (refer to 9.2.2, “IPv6 addressing” on
page 339).
3.1.5 The IP address exhaustion problem
The number of networks on the Internet has been approximately doubling
annually for a number of years. However, the usage of the Class A, B, and C
networks differs greatly. Nearly all of the new networks assigned in the late
1980s were Class B, and in 1990 it became apparent that if this trend continued,
the last Class B network number would be assigned during 1994. However,
Class C networks were hardly being used.
The reason for this trend was that most potential users found a Class B network
to be large enough for their anticipated needs, because it accommodates up to
65534 hosts, while a Class C network, with a maximum of 254 hosts, severely
restricts the potential growth of even a small initial network. Furthermore, most of
the Class B networks being assigned were small ones. There are relatively few
networks that would need as many as 65,534 host addresses, but very few for
which 254 hosts would be an adequate limit. In summary, although the Class A,
Class B, and Class C divisions of the IP address are logical and easy-to-use
(because they occur on byte boundaries), with hindsight, they are not the most
practical because Class C networks are too small to be useful for most
organizations, while Class B networks are too large to be densely populated by
any but the largest organizations.
In May 1996, all Class A addresses were either allocated or assigned, as well as
61.95 percent of Class B and 36.44 percent of Class C IP network addresses.
The terms assigned and allocated in this context have the following meanings:
► Assigned: The number of network numbers in use. The Class C figures are
somewhat inaccurate, because the figures do not include many Class C
networks in Europe, which were allocated to RIPE and subsequently
assigned but which are still recorded as allocated.
► Allocated: This includes all of the assigned networks and additionally, those
networks that have either been reserved by IANA (for example, the 63 Class
A networks are all reserved by IANA) or have been allocated to regional
registries by IANA and will subsequently be assigned by those registries.
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