Examining IP Addressing
By: E.J Gallagher, MCSE
Copyright 2001
Skip to Executive Summary ->
Let’s try this again!
And this time I’ll try to maintain a
coherent train of thought!
Intro to IP Addresses and
Subnet Masks
Let’s start with the basics
Bit
Short for binary digit, the smallest unit of information
on a machine. A single bit can hold only one of two
values: 0 or 1. More meaningful information is obtained
by combining consecutive bits into larger units. For
example, a byte is composed of 8 consecutive bits.
Byte
More Basics
Abbreviation for binary term, a unit of storage
capable of holding a single character. On almost
all modern computers, a byte is equal to 8 bits.
Large amounts of memory are indicated in
terms of kilobytes (1,024 bytes), megabytes
(1,048,576 bytes), and gigabytes
(1,073,741,824 bytes). A disk that can hold
1.44 megabytes, for example, is capable of
storing approximately 1.4 million characters, or
about 3,000 pages of information
Now let’s see how those
basics apply to IP Addresses
IP Address structure


Dotted decimal Notation
Binary Notation
Converting Binary and Decimal Notation
Subnet Mask Introduction


Default Subnet Masks
Custom Subnet masks
A typical IP address
Consists of four sets of numbers called Octets or
Bytes, separated by dots (or periods).
This is referred to as Dotted Decimal Notation.
That same IP Address can also be
represented in Binary Notation.
Where each Bit in an octet is represented by either a 0 or a 1
Decimal
Binary
In either notation, there are still 4 octets separated by dots
that make up the IP Address.
In Binary Notation:
Each octet consists of 8 bits. And each bit within
an octet (or byte) has a corresponding value.
Bit Values
Binary
128 64 32 16 8 4 2 1
1 1 1 1 1 1 1 1
High
----------------------------------------
order
bits
Low
order
bits
Converting from Binary to
Decimal Binary Notation
Since each bit in an octet has a corresponding
value…
Bit Values
Binary
Converting from Binary to
Decimal Binary Notation
…by adding together the values for ONLY
those bits in the octet that contain a 1, we
determine the decimal value of that octet.
Converting from Binary to
Decimal Binary Notation
Another example:
…from Decimal to Binary
What is the binary notation for decimal 76?
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
1
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
1
0
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
1
0
0
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
1
0
0 1
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
1
0
0 1 1
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
1
0
0 1 1 0
…from Decimal to Binary
What is the binary notation for decimal 76?
128 64 32 16 8 4 2 1
0
1
0
0 1 1 0 0
This conversion is done on
each octet to convert the full
IP address.
An IP Address is used to
Determine the Network ID
Determine the Host ID
But How?
By applying a Subnet Mask
We use a subnet mask to
determine which octets (or bits) in
the IP Address represent the
Network ID and which octets (or
bits) represent the Host ID.
A Subnet Mask
Is also a set of four octets separated by
dots.
Can also be represented in either
Dotted Decimal or Binary Notation.
But must consist of contiguous 1’s on
the left, followed by contiguous 0’s on
the right.
A Subnet Mask
Subnet mask in Dotted Decimal Notation
255.0.0.0
The same subnet mask in binary Notation
11111111.00000000.00000000.00000000
Four octets
Binary or Dotted Decimal Notation
Contiguous 1s followed by contiguous 0s
A Subnet Mask
Is applied to an IP Address to determine the
Network ID portion of the IP Address and the
Host ID portion of the IP Address.
192.168.12.32
255. 0 .0 . 0
11000000.10101000.00001100.00100000
11111111.00000000.00000000.00000000
Subnet Mask
The leftmost 1’s indicate the Network ID portion
of the IP address.
The rightmost 0’s indicate the Host ID portion of
the IP address.
IP Address
Subnet Mask
IP Address
Subnet Mask
Subnet Mask
IP Address
Subnet Mask
IP Address
Subnet Mask
Result: Network ID is 192.168.0.0
Host ID is 12.32
There are 2 kinds of Subnet
Masks
Default Subnet Mask
Custom Subnet Mask
Default Subnet Masks
Default Subnet Masks are used in Classfull
networks that have not been subnetted.
Bits used for Network ID are confined by octet
boundaries.
11111111.00000000.00000000.00000000
255 .
0
.
0
.
0
11111111.11111111.00000000.00000000
255
. 255
.
0
.
0
11111111.11111111.11111111.00000000
255
. 255
.
255 .
0
* To be discussed further in Classfull IP Addressing
Custom Subnet Masks
Custom Subnet Masks are used to
subdivide networks into smaller subnets.
Network ID borrows two or more Host bits to extend
the number possible networks (subnets).
11111111.11000000.00000000.00000000
255 . 192
.
0
.
0
11111111.11111111.11110000.00000000
255
. 255
.
240
.
0
11111111.11111111.11111111.11111100
255
. 255
.
255
. 252
* To be discussed further in Classfull IP Addressing
So now we know….
Structure of an IP Address


In both Binary and Dotted Decimal
How to convert between Binary and Decimal
The purpose of an IP Address

To identify the Network and Host ID
Structure of a Subnet Mask

In both Binary and Dotted Decimal
The purpose of a Subnet Mask

To mask the Network ID bits from the Host ID bits
So now let’s look at
Classfull IP Addressing
Classfull IP Addressing
Is a method of assigning IP Addresses (or
portions of the IP Address Space) to organizations
based on the number of Internet connected hosts
that a given organization needs to support.
Classfull IP addressing is based on 3 assignable
classes of network IDs, each class having a specific
number of network IDs, and an associated number
of Hosts per network.
The IP Address Space
How big is it?
And how to we distribute it?
InterNIC
Internet Network Information Center
Decided that an address space comprised of 32 bits
would be sufficient to handle all potential networks and
hosts that would ever be connected to the internet.
232 equals 4.3 billion different addresses.
Devised a method using address classes that allowed
for an appropriate allocation of the available address
space based on the number of hosts an organization
needed to support. Classes A,B,C,D,E were defined,
with classes D and E reserved for special use.
Classfull IP Addressing.
IP Address Space
4.3 billion different addresses
We use those 32 bits to denote an IP address
in either Binary or Decimal notation.
Classfull IP Addressing
The IP address space is divided into 5 classes. But only
3 classes are used to assign IP addresses. Each class
contains a specific number of Network Ids and Host Ids,
and each class has a specific range of values for the
first octet.
Number of
Number of
First Octet
Class Networks
Hosts/Network
Address Range
A
B
C
D
E
126
16,384
2,097,152
Reserved for
Reserved for
16,777,214
1 - 126
65,534
128 - 191
254
192 - 223
special use-multicasting
special use-experimental
Classfull IP Addressing
The class of a network is identified by the
value of the first octet in the IP address.
Class
Number of
Networks
Number of
Hosts/Network
First Octet
Address Range
A
B
C
126
16,384
2,097,152
16,777,214
65,534
254
1 - 126
128 - 191
192 - 223
Classfull IP Addressing
The class of a network is identified by the
value of the first octet in the IP address.
Class
Number of
Networks
Number of
Hosts/Network
First Octet
Address Range
A
B
C
126
16,384
2,097,152
16,777,214
65,534
254
1 - 126
128 - 191
192 - 223
Example: Class ?
Class ?
Class ?
201.112.37.45
178.47.123.6
113.126.54.12
In Classfull IP Addressing
The class of a network is identified by the
value of the first octet in the IP address.
Class
Number of
Networks
Number of
Hosts/Network
First Octet
Address Range
A
B
C
126
16,384
2,097,152
16,777,214
65,534
254
1 - 126
128 - 191
192 - 223
Example: Class C
Class B
Class A
201.112.37.45
178.47.123.6
113.126.54.12
In Classfull IP Addressing
When we know the class of the network, then we can
determine which octets in the IP address represent the
Network ID, and which octets represent the Host ID.
Class A uses the first octet to indicate the Network ID
Class B uses the first two octets for the Network ID
Class C uses the first three octets for the Network ID
The remaining octets represent the Host ID.
Yeah, but…
where do these numbers come from?
And what do they mean?
Class
Number of
Networks
Number of
Hosts/Network
First Octet
Address Range
A
126
16,777,214
1 - 126
B
16,384
65,534
128 - 191
C
2,097,152
254
192 - 223
“Oh no… here comes the math part!”
Lets look at how these
numbers are arrived at.
Range of values in the first octet.
Number of networks available in a class.
Number of Hosts available per network.
Where the range of values in the first
octet came from…
The range of values in the first octet is determined by
how the high order bits are set for each class.
High order bits are any number of leftmost bits in an octet.
These settings were defined by InterNIC to establish
the different network classes.
0 1 1 1 1 1 1 1
1 0 1 1 1 1 1 1
1 1 0 1 1 1 1 1
High order bits
In a class A network ID the high order
bit in the first octet is always set to 0.
In a class B network ID the high order
bits in the first octet are always set to
10.
In a class C network ID the high order
bits in the first octet are always set to
110.
Keep in mind, these are Bits, and bits have values…
So, for a class A network ID….
…if the high order bit in the first octet is 0 then the
possible range of values in the first octet is:
0 0 0 0 0 0 0 1 = Decimal 1 (Min.value)
0 1 1 1 1 1 1 1 = Decimal 127 (Max. value)
But remember the 127 address is reserved for the
loopback function.
So… the range of valid network addresses for Class A
is 1 thru 126.
This is the only range of values possible when the high
order bit is set to 0.
for a class B network ID….
…if the high order bits in the first octet are set to 10
then the possible range of values in the first octet is:
1 0 0 0 0 0 0 0 = Decimal 128 (Min.value)
1 0 1 1 1 1 1 1 = Decimal 191 (Max. value)
So… the range of valid network addresses in the first
octet for Class B is 128 thru 191.
This is the only range of values possible when the high
order bits are set to 10.
for a class C network ID….
…if the high order bits in the first octet are set to 110
then the possible range of values in the first octet is:
1 1 0 0 0 0 0 0 = Decimal 192 (Min.value)
1 1 0 1 1 1 1 1 = Decimal 223 (Max. value)
So… the range of valid network addresses in the first
octet for Class C is 192 thru 223.
This is the only range of values possible when the high
order bits are set to 110.
Yeah, but…
where do these numbers come from?
And what do they mean?
Class
Number of
Networks
Number of
Hosts/Network
First Octet
Address Range
A
126
16,777,214
1 - 126
B
16,384
65,534
128 - 191
C
2,097,152
254
192 - 223
How the number of networks
available in each class is determined.
Class A Network:
Uses only the first octet to represent the network ID
using only 8 bits.
If the high order bit is set to 0 then there are only 7
remaining bits to use for network IDs.
2(Remaining bits)=Number of Networks
7
2 =128 possible network Ids
But 127 is reserved, and a network ID can’t
be all 0s, so there are 126 available
Network Ids in a class A space.
How the number of networks
available in each class is determined.
Class B Network:
Uses the first two octets to represent the network ID
using 16 bits.
If the high order bits are set to 10 then there are
only 14 remaining bits to use for network IDs.
2(Remaining bits)=Number of Networks
214=16,384
possible network Ids
How the number of networks
available in each class is determined.
Class C Network:
Uses the first three octets to represent the network
ID using 24 bits.
If the high order bits are set to 110 then there are
only 21 remaining bits to use for network IDs.
2(Remaining bits)=Number of Networks
221=2,097,152
possible network Ids
Yeah, but…
where do these numbers come from?
And what do they mean?
Class
Number of
Networks
Number of
Hosts/Network
First Octet
Address Range
A
126
16,777,214
1 - 126
B
16,384
65,534
128 - 191
C
2,097,152
254
192 - 223
How the number of Hosts per
network is determined for each class.
Class A
Uses one octet for the network ID, and three octets for
the host ID, meaning 24 bits are used for the Host ID.
2(Host bits)-2=Hosts per network
224-2=16,777,214 Hosts per network
**Since the host ID cannot be all 1s (broadcast address)
or all 0s (represents a network ID).
How the number of Hosts per
network is determined for each class.
Class B
Uses two octets for the network ID, and two octets for
the host ID, meaning 16 bits are used for the Host ID.
2(Host bits)-2=Hosts per network
216-2=65,534 Hosts per network
**Since the host ID cannot be all 1s (broadcast address)
or all 0s (represents a network ID).
How the number of Hosts per
network is determined for each class.
Class C
Uses three octets for the network ID, and one octet for
the host ID, meaning 8 bits are used for the Host ID.
2(Host bits)-2=Hosts per network
28-2=254 Hosts per network
**Since the host ID cannot be all 1s (broadcast address)
or all 0s (represents a network ID).
Yeah, but…
where do these numbers come from?
And what do they mean?
Class
Number of
Networks
Number of
Hosts/Network
First Octet
Address Range
A
126
16,777,214
1 - 126
B
16,384
65,534
128 - 191
C
2,097,152
254
192 - 223
So to review…..
Range of values in the first octet.

Is determined by the values of the
high order bits.
Number of networks available in a class.

2(Remaining bits)=Number of Networks
Number of Hosts available per network.

2(Host bits)-2=Hosts per network
Now let’s take another look at default
subnet masks and how they relate to
the three IP Address classes.
Default subnet masks:
Class A 255.0.0.0
 Class B
255.255.0.0
 Class C
255.255.255.0

Default Subnet Masks
We know that each network class uses a specific number
of octets to represent the network ID, and we know the
subnet mask is used to determine which portion of the IP
address indicates the network ID, so…
A
B
C
Default Subnet Masks
…the default subnet mask for a class mirrors the octets
used for the network ID portion of the IP Address.
A
255
B
C
.
0
.
0
.
0
Default Subnet Masks
Because each network class uses a specific number of
octets to represent the network ID, and because the
subnet mask is used to indicate which portion of the IP
address indicates the network ID.
A
B
C
255
.
0
.
0
.
0
255
.
255
.
0
.
0
Default Subnet Masks
Because each network class uses a specific number of
octets to represent the network ID, and because the
subnet mask is used to indicate which portion of the IP
address indicates the network ID.
A
B
255
.
0
.
0
.
0
255
.
255
.
0
.
0
255
.
255
.
255
.
0
C
A quick quiz…………..
Determining the Network and
Host Ids using Network Class
and Default Subnet Mask
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
IP Address
Subnet Mask
Network ID
195.157.52.107
Host ID
Host ID
Host ID
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
IP Address
Subnet Mask
Network ID
131.150.22.10
IP Address
Subnet Mask
Network ID
195.157.52.107
Host ID
Host ID
Host ID
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
Determine the Network Class, Subnet
Mask, Network ID and Host Id
IP Address
Subnet Mask
Network ID
122.110.12.101
255. 0 . 0. 0
122. 0 . 0. 0 Host ID 110.12.101
IP Address
Subnet Mask
Network ID
131.150.22.10
255.255 .0 . 0.
131.150 . 0. 0
IP Address
Subnet Mask
Network ID
195.157.52.107
255.255.255. 0.
195.157. 52. 0 Host ID 107
Host ID 22.10
What we’ve seen so far….
IP Address structure
 Dotted decimal Notation
 Binary Notation
Converting Binary and Decimal Notation
Subnet Mask Introduction
 Default Subnet Masks
 Custom Subnet masks
Classfull IP Addressing
 Classes of Networks
 Formulas
 Default Subnet Masks

Determining Network and Host Ids
…all for a single network segment, no subnets or
routers.
Now let’s look at……….
Subnetting
Subnetting is the process of subdividing an
already existing network ID into multiple
smaller network IDs.
Remember we said that...
The IP Address Space is already divided into three
major categories.
Each category supports a predetermined number of
networks and hosts.
The number of network and host Ids available is a
function of the number of bits available to each
portion of the IP address.
115.168.12.32
255. 0 .0 . 0
01110011.10101000.00001100.00100000
11111111.00000000.00000000.00000000
In order to subdivide a
network (create subnets)…
We need to borrow bits from the Host
portion, and loan them to the Network
portion of the IP address.
This produces additional network Ids but
reduces the number of hosts available.
To do this we need to define a Custom
Subnet Mask which indicates how many bits
are used for the network ID.
Default vs. Custom
Default Subnet Mask
11111111.11111111.00000000.00000000
255
.
255
.
0
.
0
Custom Subnet Mask
11111111.11111111.11000000.00000000
255
.
255
.
192
.
0
When using a custom subnet mask, it is no longer
possible to determine the length of the network ID
by simply looking at the value in the first octet of
the IP Address..
Custom Subnet Mask
11111111.11111111.11000000.00000000
255
How
Why
How
How
.
255
.
192
.
many bits borrowed?
the decimal value of 192?
many subnets did we create?
many hosts on each subnet?
0
Custom Subnet Mask
11111111.11111111.11000000.00000000
255
.
255
Bits borrowed?
Decimal value?
Subnets created?
Hosts per subnet?
.
192
.
0
2
Value of bits borrowed
2
(2Bits borrowed-2)
16,382 (2Host Bits-2)
Note: A normal class B address has 65,534 hosts
available(216-2). After subnetting this network, we only
have 32,764 total hosts available (16,382 x 2)
Subnet Masks
11111111.11111111.11000000.00000000
255
.
255
.
192
.
0
11111111.11111111.11100000.00000000
255
.
255
.
224
.
0
11111111.11111111.11110000.00000000
255
.
255
.
240
.
0
11111111.11111111.11111000.00000000
255
.
255
.
248
.
0
Subnet Masks
11111111.11111111.11111100.00000000
255
.
255
.
252
.
0
11111111.11111111.11111110.00000000
255
.
255
.
254
.
0
11111111.11111111.11111111.00000000
255
.
255
.
255
.
0
Subnet Masks on other classes
11111111.11000000.00000000.00000000
255
.
192
.
0
.
0
11111111.11111111.11110000 .00000000
255
.
255
.
240
.
0
11111111.11111111.11111111.11111000
255
.
255
.
255
.
248
Formulas to remember….
Number of subnets
(bits
borrowed)
2
-2
Hosts per subnet
(Host
bits)
2
-2
But what are the actual
network Ids?
11111111.11111111.11100000.00000000
255
.
255
.
224
.
0
By borrowing 3 bits, we create 6 subnets.
23-2=6
The Network IDs start with and increment by
the value of the lowest bit borrowed.
The 32,64,96,128,160,192 subnets.



32 thru 63
64 thru 95
96 thru 127
126 thru 159
160 thru 191
192 thru 224
In this case, we’ll refer to 32 as the Address Block Start
But what are the actual
network Ids? Another example.
11111111.11111111.11000000.00000000
255
.
255
.
192
.
0
By borrowing 2 bits, we create 2 subnets.
22-2=2
The Network Ids start with and increment by
the value of the lowest bit borrowed.
The 64 subnet and the 128 subnets.

64 thru 127
64 Network ID 65 thru 127 Host IDs
128 thru 191 128 Network ID 129 thru 191 Host Ids

In this case 64 is the Address Block Start

Anding… to determine the
network ID.
137 . 107
.
187 .
10
255 . 255
.
192 .
0
10001001.01101011.10111011.00001010 IP Address
11111111.11111111.11000000.00000000 Subnet mask
10001001.01101011.10000000.00000000 Network ID
137 . 107
.
128 .
0
137 . 107
.
75 .
10
255 . 255
.
192 .
0
10001001.01101011.01000000.00001010 IP Address
11111111.11111111.11000000.00000000 Subnet Mask
10001001.01101011.01000000.00000000 Network ID
137 . 107
.
64 .
0
Anding… to determine the
network ID.
137 . 107
.
187 .
10
255 . 255
.
192 .
0
10001001.01101011.10111011.00001010
11111111.11111111.11000000.00000000
10001001.01101011.10000000.00000000
137 . 107
.
128 .
0
IP Address
Subnet mask
Network ID
Network ID
137 . 107
.
64 .
10
255 . 255
.
192 .
0
10001001.01101011.01000000.00001010 IP Address
11111111.11111111.11000000.00000000 Subnet Mask
10001001.01101011.01000000.00000000 Network ID
137 . 107
.
64 .
0
Anding… to determine the
network ID.
137 . 107
.
187 .
10
255 . 255
.
192 .
0
10001001.01101011.10111011.00001010
11111111.11111111.11000000.00000000
10001001.01101011.10000000.00000000
137 . 107
.
128 .
0
IP Address
Subnet mask
Network ID
Network ID
137 . 107
.
75 .
10
255 . 255
.
192 .
0
10001001.01101011.01001011.00001010
11111111.11111111.11000000.00000000
10001001.01101011.01000000.00000000
137 . 107
.
64 .
0
IP Address
Subnet Mask
Network ID
Network ID
Another way to determine the
Network ID…
IP Addr
137 . 107 . 187 . 10
255 . 255 . 192 . 0
Sub Mask
===============================
Net ID
137 . 107 .
?
. 0
What part of the IP Addr are we solving for?
What is the Addr Block Start?
Another way to determine the
Network ID…
IP Addr
137 . 107 . 187 . 10
255 . 255 . 192 . 0
Sub Mask
===============================
Net ID
137 . 107 .
?
. 0
What part of the IP Addr are we solving for? 187
What is the Addr Block Start? 64 (Value of lowest bit borrowed)
Solve Addr / ABS = Mult * ABS = Net ID
Drop any
Decimals
Another way to determine the
Network ID…
IP Addr
137 . 107 . 187 . 10
255 . 255 . 192 . 0
Sub Mask
===============================
Net ID
137 . 107 . 128 . 0
What part of the IP Addr are we solving for? 187
What is the Addr Block Start? 64
Solve Addr / ABS = Mult * ABS = Net ID
187 / 64 = 2 * 64 = 128
…and determine the range of
addresses within that subnet.
IP Addr
137 . 107 . 187 . 10
255 . 255 . 192 . 0
Sub Mask
===============================
Net ID
137 . 107 . 128
. 0
What part of the IP Addr are we solving for? 187
What is the Addr Block Start? 64
Solve Addr / ABS = Mult * ABS = Net ID
187 / 64 = 2 * 64 = 128
187 / 64 = 2 x 64 = 128 + (64-1) = 191
Tom’s question…
How can you have a class A address with a
class B subnet mask?
Answer……
A class A address with a class B subnet mask is
simply a class A network subnetted by
borrowing 8 bits from the host ID.
115 . 168 .12 . 32
255 . 255 .0 . 0
01110011.10101000.00001100.00100000
11111111.11111111.00000000.00000000
Default Mask
Borrowed Bits
Tom’s question…
How can you have a class A address with a
class B subnet mask?
115 . 168 .12 . 32
255 . 255 .0 . 0
01110011.10101000.00001100.00100000
11111111.11111111.00000000.00000000
Default Mask
Borrowed Bits
8 borrowed bits 28-2= 254 Subnets
16 Host bits 216-2 = 65,534 Hosts / Subnet
Subnetting Benefits
What do we gain by subnetting a
network?
Subnetting
Allows networks that use different
media access to be interconnected.
Increases the effective bandwidth of
each network by reducing broadcast
traffic.
Allows physically isolated networks to
communicate with each other. (WAN)
5 steps in subnetting a network:
Determine number of network Ids required.
(Plan for growth)
Determine number of hosts per subnet
required. (Plan for growth)
Define a subnet mask that will support the
network and host Ids required.
Define the network or subnet Ids to be used.
Define each of the host Ids to be used on
each subnet.
But to do that requires a
whole lot of math!
Yeah, it is, but check this out….
Limitations of Classful IP
addressing.
Permits very few variations in network sizes.
Depletion of class B addresses

2000 hosts req’d Class B= 65,534 hosts
Filling up of router tables



2000 hosts req’d Use 8 class Cs
8 x 254 = 2032
Eight routes on internet routers
Eventual depletion of all IP adresses

If Classful Addressing was still used
CIDR
Classless Inter-Domain Routing
A newer method for assigning network
Ids to an organization.
Allows for organizations to obtain IP
addresses in numbers much closer to
what they actually require.
CIDR
Classless Inter-Domain Routing
Developed for breaking up networks
into a larger variety of sizes.
Uses binary notation rather than
decimal notation.
Uses CIDR (or Network Prefix) notation.
Allows greater flexibility in creating
network addresses.
Few IP addresses go unused.
CIDR
CIDR notation involves specifying a
dotted decimal notation with a bit
mask.
10 . 217 . 123 . 7 /20
The bit mask represents the number
of bits in the subnet mask used for
the network ID.
CIDR
Windows 2000 does not accept CIDR
notation.
In Windows 2000 you must type the
IP address and subnet mask in dotted
decimal notation.
CIDR
Analyzing an IP address in CIDR notation.
CIDR Notation
10 . 217 . 123 . 7 /20
CIDR
Analyzing an IP address in CIDR notation.
CIDR Notation
10 . 217 . 123 . 7 /20
Binary Notation
00001010 . 11011001 . 01111011 . 00000111
11111111 . 11111111 . 11110000 . 00000000
CIDR
Analyzing an IP address in CIDR notation.
CIDR Notation
10 . 217 . 123 . 7 /20
Binary Notation
00001010 . 11011001 . 01111011 . 00000111
11111111 . 11111111 . 11110000 . 00000000
Dotted Decimal
10 . 217 . 123 . 7
255 . 255 . 240 . 0
CIDR
Analyzing an IP address in CIDR notation.
CIDR Notation
10 . 217 . 123 . 7 /20
Binary Notation
00001010 . 11011001 . 01111011 . 00000111
11111111 . 11111111 . 11110000 . 00000000
Dotted Decimal
10 . 217 . 123 . 7
255 . 255 . 240 . 0
What is the network ID for the above IP Address
in dotted decimal notation?
CIDR
Analyzing an IP address in CIDR notation.
CIDR Notation
10 . 217 . 123 . 7 /20
Binary Notation
00001010 . 11011001 . 01111011 . 00000111
11111111 . 11111111 . 11110000 . 00000000
00001010 . 11011001 . 01110000 . 00000000
Dotted Decimal
Network ID
10 . 217 . 123 . 7
255 . 255 . 240 . 0
10 . 217 . 112 . 0/20
CIDR
As with classful addressing, the number of hosts
available is:
(Host
bits)
2
-2
10 . 217 . 112 . 0/20
32-20 = 12 host bits
212-2= 4094
Supernetting
Supernetting is a strategy that
combines multiple addresses from the
classful environment into a single
network addresses in the classless
environment.
CIDR combines multiple classful
network Ids into a single CIDR network
ID.
Classful vs. Classless Addressing
Classful IP Addressing
Uses 3 classes with fixed octets to
represent the network ID.
Classless (CIDR) IP Addressing
Not restricted by classes, can use any
number of contiguous bits to represent
the network ID. Wider range of
assignable network Ids.
Executive Summary
The following slides are cheat sheets for
those MCSE’s with dementia.
…and determine the range of
addresses within that subnet.
IP Addr
137 . 107 . 187 . 10
255 . 255 . 192 . 0
Sub Mask
===============================
Net ID
137 . 107 . 128
. 0
What part of the IP Addr are we solving for? 187
What is the Addr Block Start? 64
Solve Addr / ABS = Mult * ABS = Net ID
187 / 64 = 2 * 64 = 128
187 / 64 = 2 x 64 = 128 + (64-1) = 191