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108 Chapter 3: OSI Reference Model & Layered Communication

With that in mind, most network layer (Layer 3) addressing schemes were created with the
following goals:

The address space should be large enough to accommodate the largest network for which
the designers imagined the protocol would be used.

The addresses should allow for unique assignment so that little or no chance of address
duplication exists.

The address structure should have some grouping implied so that many addresses are
considered to be in the same group.

In some cases, dynamic address assignment is desired.

A great analogy for this concept of network addressing is the addressing scheme used by the
U.S. Postal Service. Instead of getting involved with every small community's plans for what
to name new streets, the Post Office simply has a nearby office with a ZIP code. The rest of the
post offices in the country are already prepared to send mail to new businesses and residences
on the new streets; they care only about the ZIP code, which they already know. It is the local
postmaster's job to assign a mail carrier to deliver and pick up mail on those new streets. There
may be hundreds of Main Streets in different ZIP codes, but as long as there is just one per ZIP
code, the address is unique--and with an amazing percentage of success, the U.S. Postal
Service delivers the mail to the correct address.

Example Layer 3 Address Structures

Each Layer 3 address structure contains at least two parts. One (or more) part at the beginning
of the address works like the ZIP code and essentially identifies the grouping. All instances of
addresses with the same value in these first bits of the address are considered to be in the same
group--for example, the same IP subnet or IPX network or AppleTalk cable range. The last part
of the address acts as a local address, uniquely identifying that device in that particular group.
Table 3-10 outlines several Layer 3 address structures.

Table 3-10

Layer 3 Address Structures

Protocol

Size of Address
(Bits)

Name and Size of
Grouping Field

Name and Size of
Local Address Field

Network or subnet
(variable, between 8
and 30 bits)

Host (variable, between
2 and 24 bits)

IPX

Network (32)

Node (48)

ch03.fm Page 108 Monday, March 20, 2000 4:58 PM