Public and Private IP Addresses

Some networks connect to each other through the Internet, whereas others are private.Public and private IP addresses are required, therefore, for both of these network types.Internet stability depends directly on the uniqueness of publicly used network addresses.Therefore, some mechanism is needed to ensure that addresses are, in fact, unique. This responsibility originally rested within an organization known as the InterNIC (Internet Network Information Center). This organization was succeeded by the Internet Assigned
Numbers Authority (IANA). IANA carefully manages the remaining supply of IP addresses to ensure that duplication of publicly used addresses does not occur. Such duplication would cause instability in the Internet and compromise its capability to deliver datagrams to networks using the duplicated addresses.To obtain an IP address or block of addresses, you must contact an Internet service provider (ISP). The ISP allocates addresses from the range assigned by their upstream registry or their appropriate regional registry, which is managed by IANA, as follows:
■ Asia Pacific Network Information Center (APNIC)
■ American Registry for Internet Numbers (ARIN)
■ Réseaux IP Europens Network Coordination Centre (RIPE NCC)
With the rapid growth of the Internet, public IP addresses began to run out, so new
addressing schemes such as classless interdomain routing (CIDR) and IPv6 were developed
to help solve the problem. CIDR and IPv6 are discussed later in this chapter in the “Address
Exhaustion” section.
Although Internet hosts require a globally unique IP address, private hosts that are not connected to the Internet can use any valid address, as long as it is unique within the private network. Because many private networks exist alongside public networks, grabbing “just any address” is strongly discouraged. Therefore, the IETF defined 3 blocks of IP addresses (1 Class A network, 16 Class B networks, and 256 Class C networks) in RFC 1918 for private, internal use. Addresses in this range are not routed on the Internet backbone, as shown in Table 1-2. Internet routers are configured to discard private addresses as defined
by RFC 1918.
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 havepredicted 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.
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.
Table 1-2 Private IP Addresses
Class RFC 1918 Internal Address Range
A 10.0.0.0 to 10.255.255.255
B 172.16.0.0 to 172.31.255.255
C 192.168.0.0 to 192.168.255.255