chapter 4

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Hands
-
On Microsoft
Windows Server 2003
Networking


Chapter Four

Subnetting


2

Objectives


Understand why subnetting is necessary


Describe how computers work with binary
TCP/IP addresses


Subnet any network


Determine the number of useable hosts on a
subnet


Supernet several smaller networks


3

Reasons to Subnet


Subnetting


The process of separating a network into several
smaller networks to improve performance


Subnetting is used because it can


Reduce collisions


Limit broadcasts


Control traffic

4

Reducing Collisions


Collision


When two computers on an Ethernet network
using CSMA/CD as an access method attempt to
transmit at the same time


Subnetting


Reduces number of hosts on each network


Reduces the amount of traffic on the network


With less traffic, collision is reduced


Collisions that occur on one network do not
affect another network

5

Limiting Broadcasts


Subnetting a network


Creates multiple networks with fewer hosts on
each network


Fewer hosts results in fewer broadcast messages


Routers


Packets addressed to IP address
255.255.255.255 are not forwarded


6

Controlling Traffic


Routers


Allows a greater degree of control over network
traffic


Most have ability to implement rules about which
packets they forward



7

Binary and TCP/IP


IP addresses


Internally, computer looks at an IP address as a
single group of 32 binary digits


Decimal numbering


A base
-
ten numbering system


Uses the values from
0

to
9

for each digit


8

Counting in Decimal

9

Decimal Column Values

10

Binary Numbering


Subnetting


Based on binary


Binary


Base two numbering system


Two potential values for each digit:
0
and
1

11

Binary Counting

12

Binary Column Values

13

Conversion Between Binary and
Decimal


To convert a binary octet to a decimal value


Multiply the digit in each column by the value of
each column


Then determine the sum of those products


14

Binary to Decimal Conversion

15

Windows Calculator in Scientific
Mode

16

Binary Subnet Mask


Computer calculates the host ID and network ID
of an IP address while working in binary


Part of the network ID


Where there is a
1

in the subnet mask


Part of the host ID


Where there is a
0

in the subnet mask


ANDing


Binary process used by computers to find the
network ID


An IP address ANDed with a subnet mask gives
the network ID


1s in a subnet mask must be contiguous

17

Calculating Host ID and Network ID

18

Invalid Subnet Masks

19

Subnetting a Network


To subnet a network



Take some bits from the host ID and give them to
the network ID


Class B addresses


Normally very large


Generally need to be subnetted to handle routing
between different physical locations


Using an entire octet for subnetting gives 256
possible subnets


Traditionally, subnets with all
1
s and all
0
s are
discarded, leaving 254 usable subnets

20

Simple Subnetting

21

Complex Subnetting

22

Subnet Hosts


2
n
-
2
, where
n

is the number of host bits



Formula used to calculate the number of subnets
that can be created from a certain number of bits


Also used to calculate the number of usable hosts
on a subnet


23

Usable Hosts

24

Usable Hosts Formula

25

Supernetting


The opposite of subnetting


Used to create one large network from several
smaller ones


Takes bits from network ID and gives them to
the host ID


All networks being combined for supernetting
must be contiguous


IP addresses from first network to last must be
one single range with no breaks

26

Supernetting Two Class C Networks

27

Supernetting Four Class C Networks

28

Reasons for Supernetting


Supernetting


Used when a range of IP addresses larger than a
Class C network is required, but a full Class B
network is not required


May also be used to reduce routing complexity




29

Summary


Subnetting


Used to divide a single large network into multiple
smaller networks


Reduces packet collisions


Limits broadcasts


Controls network traffic


Binary


Base
-
two numbering system


Only
0

and
1

are valid values


Computers


Work with IP addresses as 32
-
digit binary
numbers

30

Summary (Continued)


Subnet Mask


1 corresponds with bit that is part of network ID


0 corresponds with bit that is part of host ID



Subnetting


Takes bits from host ID and uses them as part of
network ID


Number of useable hosts on a network


Calculated with the formula 2
n
-
2


n
is the number of host bits


Supernetting


Combines multiple smaller networks into a single
larger network


Networks being combined must be contiguous