system attached to an IP-based internet be assigned a unique, 32-bit Internet address
value. Some systems, such as routers which have interfaces to more than one network,
must be assigned a unique IP address for each network interface.
The first part of an Internet address identifies the network on which the host resides,
while the second part identifies the particular host on the given network. This created the
two-level addressing hierarchy which is illustrated in Figure 3.
Figure 3: Two-Level Internet Address Structure
In recent years, the network-number field has been referred to as the "network-prefix"
because the leading portion of each IP address identifies the network number. All hosts
on a given network share the same network-prefix but must have a unique host-number.
Similarly, any two hosts on different networks must have different network-prefixes but
may have the same host-number.
Primary Address Classes
In order to provide the flexibility required to support different size networks, the
designers decided that the IP address space should be divided into three different address
classes - Class A, Class B, and Class C. This is often referred to as "classful"
addressing because the address space is split into three predefined classes, groupings, or
categories. Each class fixes the boundary between the network-prefix and the hostnumber
at a different point within the 32-bit address. The formats of the fundamental
address classes are illustrated in Figure 4.
One of the fundamental features of classful IP addressing is that each address contains a
self-encoding key that identifies the dividing point between the network-prefix and the
host-number. For example, if the first two bits of an IP address are 1-0, the dividing
point falls between the 15th and 16th bits. This simplified the routing system during the
early years of the Internet because the original routing protocols did not supply a
"deciphering key" or "mask" with each route to identify the length of the network-prefix.
Class A Networks (/8 Prefixes)
Each Class A network address has an 8-bit network-prefix with the highest order bit set
to 0 and a seven-bit network number, followed by a 24-bit host-number. Today, it is no
longer considered 'modern' to refer to a Class A network. Class A networks are now
referred to as "/8s" (pronounced "slash eight" or just "eights") since they have an 8-bit
network-prefix.
A maximum of 126 (27-2) /8 networks can be defined. The calculation requires that the
2 is subtracted because the /8 network 0.0.0.0 is reserved for use as the default route and
the /8 network 127.0.0.0 (also written 127/8 or 127.0.0.0/8) has been reserved for the
"loopback" function. Each /8 supports a maximum of 16,777,214 (224-2) hosts per
network. The host calculation requires that 2 is subtracted because the all-0s ("this
network") and all-1s ("broadcast") host-numbers may not be assigned to individual
hosts.
Since the /8 address block contains 231 (2,147,483,648 ) individual addresses and the
IPv4 address space contains a maximum of 232 (4,294,967,296) addresses, the /8
address space is 50% of the total IPv4 unicast address space.
Class B Networks (/16 Prefixes)
Each Class B network address has a 16-bit network-prefix with the two highest order
bits set to 1-0 and a 14-bit network number, followed by a 16-bit host-number. Class B
networks are now referred to as"/16s" since they have a 16-bit network-prefix.
A maximum of 16,384 (214) /16 networks can be defined with up to 65,534 (216-2)
hosts per network. Since the entire /16 address block contains 230 (1,073,741,824)
addresses, it represents 25% of the total IPv4 unicast address space.
Class C Networks (/24 Prefixes)
Each Class C network address has a 24-bit network-prefix with the three highest order
bits set to 1-1-0 and a 21-bit network number, followed by an 8-bit host-number. Class
C networks are now referred to as "/24s" since they have a 24-bit network-prefix.
A maximum of 2,097,152 (221) /24 networks can be defined with up to 254 (28-2)
hosts per network. Since the entire /24 address block contains 229 (536,870,912)
addresses, it represents 12.5% (or 1/8th) of the total IPv4 unicast address space.
Other Classes
In addition to the three most popular classes, there are two additional classes. Class D
addresses have their leading four-bits set to 1-1-1-0 and are used to support IP
Multicasting. Class E addresses have their leading four-bits set to 1-1-1-1 and are
reserved for experimental use.
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