Information Technology Reference
In-Depth Information
The 32-bit IP address is a structured, or hierarchical, address as opposed to a flat, or
nonhierarchical, address. Although IP could have used either
flat addressing
or
hierarchical addressing
, its designers elected to use the latter for a very good reason, as you
will now see.
why hierarchical addressing Is used
What's the difference between flat and hierarchical addressing? A good example of a flat
addressing scheme is a U.S. state driver's license number. There's no partitioning to it;
the range of legal numbers isn't broken up in any meaningful way (say, by county of resi-
dence or date of issue). If this method had been used for IP addressing, every machine
on the Internet would have needed a totally unique address, just as each driver's license
number in a particular state is unique.
The good news about flat addressing is that it can handle a large number of addresses
in 32 bits of data, namely, 4.3 billion. A 32-bit address space with two possible values for
each position—either 0 (zero) or 1 (one)—gives you 2
32
values, which equals approxi-
mately 4.3 billion.
The bad news—and the reason flat addressing isn't used in IP—relates to routing. If every
address were totally unique, every router on the Internet would need to store the address
of every other machine on the Internet. It would be fair to say that this would make effi-
cient routing impossible, even if only a fraction of the possible addresses were used.
The solution to this dilemma is to use a hierarchical addressing scheme that breaks the
address space into ordered chunks. Telephone numbers are a great example of this type
of addressing. The first section of a U.S. telephone number, the area code, designates
a very large area. The area code is followed by the prefix, which narrows the scope to a
local calling area. The final segment, the customer number, zooms in on the specific con-
nection. By looking at a number such as 603-766-xxxx, you can quickly determine that the
number is located in the southern part of New Hampshire (area code 603) in the Ports-
mouth area (the 766 exchange).
IP Address Structure
IP addressing works the same way. Instead of the entire 32 bits being treated as a unique
identifier, one part of the IP address is designated as the network address (or network ID)
and the other part as a node address (or host ID), giving it a layered, hierarchical structure.
Together, the IP address, the network address, and the node address uniquely identify a
device within an IP network.
The network address—the first two sets of numbers in an IP address—uniquely
identifies each network. Every machine on the same network shares that network address
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