Sidebar: TCP/IP Addresses
One of the most commonly used IP network addresses
today is 192.1.0.0.
Why? Because it, or a similar address, is often used
in configuration
examples found in many hardware and software manuals.
These
configurations are often implemented "as is"
with no modification either
because the site administrator is unfamiliar with IP
address
registration, or because it is assumed that the configured
systems will
never be attached to the worldwide Internet. It is interesting
to note
that the address mentioned above, as well as others
like 192.x.0.0 and
192.0.x.0 are valid addresses on the Internet.
You may have noticed that the addresses 192.168.65.0
and 192.168.81.0
are used in this article. These addresses were not just
pulled out of
thin air. They, as well as quite a few others, have
been reserved by the
Internet Assigned Numbers Authority (IANA) for use by
any organization
on its internal networks. RFC1597 documents the reservation
of the
following addresses:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.0.0
192.168.0.0 - 192.168.255.255
As RFC1597 notes "the first block is nothing but
a single class A
network number, while the second block is a set of 16
contiguous class B
network numbers, and the third block is a set of 255
contiguous class C
network numbers." The RFC continues:
"An enterprise that decides to use IP addresses
out of the address space
defined in this document [RFC1597] can do so without
any coordination
with IANA or an Internet registry. Addresses with this
private address
space will only be unique within the enterprise."
Organizations that decide to use these addresses on
their internal
segments have the flexibility to implement arbitrarily
large networks
without having to worry about readdressing everything
if/when Internet
attachment is implemented. Additionally, a measure of
network security
is ensured by using these addresses, because hosts so
addressed are
inaccessible from direct communication with the worldwide
Internet.
(RFC1597 is available via ftp from rs.internic.net as
well as a number
of other sites.)
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