The Hierarchical IP Addressing Scheme

The Hierarchical IP Addressing Scheme
An IPv4 address is made up of 32 bits of information. These are divided into four sections,
referred to as
octets
or
bytes
, containing one byte (eight bits) each. You can depict an IP address
using three methods:

Dotted-decimal, as in 172.16.30.56

Binary, as in 10101100.00010000.00011110.00111000

Hexadecimal, as in AC 10 1E 38
All of these examples represent the same IP address. Although hexadecimal is not used
as often as dotted-decimal or binary when IP version 4 addressing is discussed, you might
find an IP address stored as hexadecimal in some programs. The 32-bit IP address is a
structured, or hierarchical, address. Although an unstructured type of addressing scheme,
like that of MAC addresses, could have been used, the structured variety was chosen for a
good reason.
The advantage of the hierarchical scheme is that it can, in a single address, represent both
the network on which a device resides and a unique identity for that device on that network.
Unstructured addresses can only hope to be unique throughout the network, but not to represent
where on the network they reside. With an unstructured address scheme, all routers on
the Internet would need to store the address of each and every interface on the Internet. This
would make efficient routing impossible, even if only a fraction of the possible addresses were
used. The use of a network portion that can be tracked separately from the host portion
allows smaller routing tables than would be possible by tracking each individual address
in its entirety.
The solution to this unstructured address dilemma is to use a two- or three-level hierarchical
addressing scheme that is structured by network and host or by network, subnet, and host. An
unstructured address is similar to how a Social Security number works in that you may know
when the number was issued and where the individual resided when that unique number was
assigned to them, but the number is no help, by itself, in locating them today.
A two- or three-level hierarchical address is comparable to the sections of a telephone number.
The first section, the area code, designates a very large area. The second section, the prefix,
narrows the scope to a local calling area. The final segment, the customer number, zooms in on
the specific subscriber. IPv4 addresses use the same type of layered structure. Rather than all
32 bits being treated as a unique identifier, as in MAC addressing, one part of the address is designated
as the network address, and the other part is designated as either the subnet and host
addresses or just the host address, if the default mask is used. Note that in some literature, the
host address
may be referred to as the
node address
.
In the following sections, we will discuss network addressing and the three different
address classes:

Class A addresses

Class B addresses

Class C addresses