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Table 2.9. Usable IPv4 Address Spaces, Expressed Classfully
Class Range Number of Nets Number of Hosts per Net
A 1.0.0.0 to 126.254.254.254 126 16,000,000
B 128.0.0.0 to 191.254.254.254 16,000 64,000
C 192.0.0.0 to 223.254.254.254 2,000,000 256
D 224.0.0.0 to 239.0.0.0 NA multicast
E 240.0.0.0 to 255.0.0.0 NA test
127.0.0.0 to 127.254.254.254 NA loopback
255.255.255.255 NA broadcast
The loopback address (127.0.0.x) is used for IP loopback testing. The host interface
sends datagrams from itself (using its assigned IP address) to itself (using the
loopback address). The loopback address is also used in some applications as a way
to internally access the network interface through IP. The first address of the IP
subnet address space—192.160.1.0, or the "zero" address—is also known as the
network loopback or network address. This address cannot be used as a host
address.
The broadcast address (255.255.255.255) is the address used by IP to send
broadcast messages to all stations. The broadcast of all ones or 255.255.255.255 is
a broadcast only to the local subnet. Routers do not forward IP broadcasts. The last
address of the IP subnet address space, 192.160.1.255, is known as the network
broadcast address. Like the network loopback address, this address cannot be used
as a host address.
The multicast address is used for IGMP messages and other multicast type
broadcasts.
Public and Unregistered IP Address Space
Because the IP address space is used to address all the hosts on the Internet, and
each host must have its own distinct address, networks that are going to connect to
the Internet need to use publicly registered IP addresses. These addresses were
assigned by the Internet Assigned Numbers Authority (IANA) until 1999, when it
was replaced by a Commerce Department unit called the Internet Corporation for
Assigned Names and Numbers (ICANN). To be on the Internet, you must apply to
one of the ICANN- designated entities for address space or get address space from
your Internet service provider (ISP). If your network is not on the Internet, you can
use any address space you want. However, because you never know whether you