C to

If you are addressing a nonpublic intranet, these private addresses can be used instead of globally unique addresses. If you want to connect a network using private addresses to the Internet, however, it is necessary to translate the private addresses to public addresses. This translation process is referred to as Network Address Translation (NAT). A router is often the network device that performs NAT.

Address Exhaustion

The growth of the Internet has resulted in enormous demands for IP addresses. This section describes the capabilities of IPv4 in relation to that demand.

When TCP/IP was first introduced in the 1980s, it relied on a two-level addressing scheme, which at the time offered adequate scalability. The architects of TCP/IP could not have predicted that their protocol would eventually sustain a global network of information, commerce, and entertainment. Twenty years ago, IPv4 offered an addressing strategy that, although scalable for a time, eventually resulted in an inefficient allocation of addresses.

The 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 B addresses, although they account for only 12.5 percent of the possible 4 billion IP addresses, as shown in Figure 1-36.

Unfortunately, Class C addresses are limited to 254 hosts, which does not meet the needs of larger organizations that cannot acquire a Class A or B address.

Figure 1-36 IP Address Allocation

As early as 1992, the IETF identified two specific concerns:

■ The Class B address category was on the verge of depletion, and the remaining, unassigned IPv4 network addresses were nearly depleted at the time.

■ As more Class C networks came online to accommodate the rapid and substantial increase in the size of the Internet, the resulting flood of new network information threatened the capability of Internet routers to cope effectively.

Over the past 20 years, numerous extensions to IPv4 have been developed to improve the efficiency with which the 32-bit address space can be used.

In addition, an even more extendable and scalable version of IP, IPv6, has been defined and developed. An IPv6 address is a 128-bit binary value, which can be displayed as 32

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