TCP/IP Tutorial and Technical Overview - IBM Redbooks

The current rules are in RFC 2050, which updates RFC 1466. The reasons for
the rules for the allocation of Class C network numbers will become apparent in
the following sections. The use of Class C network numbers in this way has
averted the exhaustion of the Class B address space, but it is not a permanent
solution to the overall address space constraints that are fundamental to IP. We
discuss a long-term solution in Chapter 9, “IP version 6” on page 327.
3.1.6 Intranets: Private IP addresses
Another approach to conserve the IP address space is described in RFC 1918.
This RFC relaxes the rule that IP addresses must be globally unique. It reserves
part of the global address space for use in networks that do not require
connectivity to the Internet. Typically these networks are administered by a
single organization. Three ranges of addresses have been reserved for this
purpose:
► 10.0.0.0: A single Class A network
► 172.16.0.0 through 172.31.0.0: 16 contiguous Class B networks
► 192.168.0.0 through 192.168.255.0: 256 contiguous Class C networks
Any organization can use any address in these ranges. However, because these
addresses are not globally unique, they are not defined to any external routers.
Routers in networks not using private addresses, particularly those operated by
Internet service providers, are expected to quietly discard all routing information
regarding these addresses. Routers in an organization using private addresses
are expected to limit all references to private addresses to internal links. They
should neither externally advertise routes to private addresses nor forward IP
datagrams containing private addresses to external routers.
Hosts having only a private IP address do not have direct IP layer connectivity to
the Internet. All connectivity to external Internet hosts must be provided with
application gateways (refer to “Application-level gateway (proxy)” on page 798),
SOCKS (refer to 22.5, “SOCKS” on page 846), or Network Address Translation
(NAT), which is discussed in the next section.
3.1.7 Network Address Translation (NAT)
This section explains Traditional Network Address Translation (NAT), Basic
NAT, and Network Address Port Translation (NAPT). NAT is also known as IP
masquerading. It provides a mapping between internal IP addresses and
officially assigned external addresses.
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