Note
You
can find more information about link-local addresses by reading RFC 3927,
“Dynamic Configuration of IPv4 Link-Local Addresses,” at http://www.ietf.org/rfc/rfc3927.txt.
RFC 3330, “Special-Use IPv4
Addresses,” is available at http://www.ietf.org/rfc/rfc3330.txt.
Legacy IPv4
Addressing
In
the early 1980s, the IPv4 addressing range was divided into three different
classes: class A, class B, and class C. Each class of addresses represented
networks of a specific fixed size. At that time in the development of IP, there
were no subnet masks to specify the net-work and host portion of the addresses.
To distinguish between the network sizes, each of these classes of addresses
was assigned address ranges. Devices could examine the high-order address to
determine how many network bits were used to define the network. For example,
for the address 192.168.2.2, because this address is in the class C addressing
range, a network device recognized this as a class C network and identified the
standard class C prefix of /24.
In
the late 1980s and early 1990s, the subnet mask was added to the IPv4
addressing scheme to allow these fixed-size networks to be subdivided or
subnetted. However, many of the restrictions of these classes remained.
By
the mid-1990s, most of the restrictions of this class-based addressing system
had been removed from the standards and the equipment operation. However, the
associated practices developed over the decade perpetuated this classful
system. Even today, some remnants of this addressing system still affect
network practices and operation. For this reason, you should be familiar with
these network classes. Table 6-13 summarizes the address classes.
Table 6-13 IPv4 Network
Classes
|
|
|
|
Number of
|
Number of
|
|
Address
|
First
|
Prefix
|
Possible
|
Hosts per
|
|
Class
|
Octet Range
|
and Mask
|
Networks
|
Network
|
|
|
|
|
|
|
|
|
|
|
|
|
|
A
|
1 to 127
|
/8 255.0.0.0
|
126 (27)
|
16,777,214
|
|
|
|
|
|
(224–2)
|
|
|
|
|
|
|
|
B
|
128 to 191
|
/16 255.255.0.0
|
16,384 (214)
|
65,534 (216–2)
|
|
|
|
|
|
|
|
C
|
192 to 223
|
/24 255.255.255.0
|
2,097,159 (221)
|
254 (28–2)
|
|
|
|
|
|
|
Historic
Network Classes
RFC
1700 defined the unicast ranges class A, class B, and class C addresses into specific
sizes. It also defined class D (multicast) and class E (experimental)
addresses, as previously
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