SOme basic terms
- Bit – A bit is a single digit with a value of 0 or 1.
- Byte – A byte is composed of 8 bits.
- Octet – An octet is also made up of 8 bits. Throughout this chapter the terms byte and octet are interchangeable.
- Network Address – This refers to a remote network in terms of routing. All hosts in the remote network fall within this address. For example, 10.0.0.0, 172.16.0.0 and 192.168.1.0
- Broadcast Address – This is the address used to send data to all hosts in a network. The broadcast address 255.255.255.255 refers to all hosts in all networks while an address such as 192.168.1.255 refers to all hosts in a particular network.
An IP address is 32 bits in
length. To make the address easier to read, it is divided into four sections of
8 bits each divided by a period. Each section is therefore, 1 byte (also called
octet) long. To further make it easier to read and remember, the binary numbers
are converted to decimal. For example, an IP address such as
11000000100000000000110000000001 is divided to make it
11000000.10000000.00001100.00000001. When this address is converted to decimal,
it will become 192.128.12.1. This format of IP address is called the dotted
decimalformat. Some applications also covert the address to hexadecimal format
instead of decimal format. However this is not commonly seen and as far as the
CCNA exam is concerned, you need to only work with the dotted decimal format.
Topics in this chapter require
binary to decimal conversions. Table 2-1 shows the decimal value of each bit
location in a byte. To easily convert from binary to decimal, add up the
decimal value corresponding to the bit place that is “on” (1). For example, a
binary value of 10110000 can be easily converted to decimal by adding the
decimal value of each bit that is 1. That gives us 128+32+16 = 176.
Table shows the decimal
value for the most common binary numbers you will encounter in this chapter.
Table Decimal
Value for each bit place in a byte
|
128
|
64
|
32
|
16
|
8
|
4
|
2
|
1
|
Table Decimal
Values for common binary numbers
|
Binary Value
|
Decimal Value
|
|
10000000
|
128
|
|
11000000
|
192
|
|
11100000
|
224
|
|
11110000
|
240
|
|
11111000
|
248
|
|
11111100
|
252
|
|
11111110
|
254
|
|
11111111
|
255
|
An IP address does not only
represent the host address. In fact it represents the network where the host
resides and the host it self. In effect, the IP address consists of two parts:
1. The
Network component – Defines network (or subnet), in an internetwork,
the host resides in.
2. The
Host component – Defines the host itself in the network.
Each combination of the
network component and the host component should be unique in the entire
Internetwork. To make it easy to identify which portion of the address is
network component and which one is the host component, addresses are broken
down into 5 classes discussed below:
- Class A – The first byte (8 bits) is the network component and the remaining three bytes (24 bits) are host component (network.host.host.host). This class is for an internetwork with small number of networks and large number of hosts per network.
- Class B – The first two bytes (16 bits) are the network component and the remaining three bytes are host components (network.network.host.host). This class bridges the gap between Class A and Class C by providing for medium number of networks with medium number of hosts.
- Class C – The first three bytes (24 bits) are the network component and the last byte (8 bits) is the host components (network.network.network.host). This class provides for large number of networks with fewer hosts per network.
- Class D – Used for multicasting.
- Class E – Reserved addresses
In a binary address the
first 5 bits of the address and the first octet in a dotted decimal address
shows the class of address. Table 2-3 shows the first 5 bits and the first
octet range of each class of address.
Table Address
range for different classes of address
|
Class
|
First 5 bits in binary
|
First Octet range
|
|
A
|
0xxxx
|
0-127 (actually 1-126
because 0 and 127 are reserved
|
|
B
|
10xxx
|
128-191
|
|
C
|
110xx
|
192-223
|
|
D
|
1110x
|
224-239
|
|
E
|
1111x
|
240-254
|
Notice that first few bits
in each class have a fixed value. For example a class A address should have the
first bit set to 0. Similarly class C should have first 2 bits set to 1 and the
third bit set to 0. Another point to note is that though the class A range is
from 0 to 127, the address 0.0.0.0 is reserved to mean “any network” and
127.0.0.1 is reserved as a loopback address which refers to the host itself. So
the class A network is restricted to the 1-126 range.
Before moving ahead, spend
some time to figure out the class of some addresses given below. Also try to
figure out which portion is the network and which portion is the host part:
1. 9.140.2.87 –
This is a Class A address because the first octet lies in 1-126 range. 9 is the
network part while 140.2.87 is the host part because class A addresses have a
network.host.host.host format.
2. 172.30.4.190 –
This is a Class B address because the first octet lies in 128-191 range. 172.30
is the network part while 4.190 is the host part because class B addresses have
a network.network.host.host format.
3. 194.144.5.10 –
This is a Class C address because the first octet lies in the 192-223 range.
194.144.5 is the network part while 10 is the host part because class C
addresses have a network.network.network.host format.
4. 45.22.187.1 –
This is again a class A address with 45 being the network part and 22.187.1
being the host part.
Some IP address such as
127.0.0.1 have a special meaning. Table 2-4 lists such addresses and what they
represent.
Table Reserved IP
addresses
|
Address
|
What it represents
|
Where can it be used
|
|
Network address of all 0s
|
Represents “this
network”. For example 0.0.0.120
|
For sending broadcast
messages to the network.
|
|
Network address of all 1s
|
Represents “all
networks”.
|
For sending broadcast
messages to all networks.
|
|
Node address of all 0s
|
Represents a network
address or all hosts in the network. Example 10.0.0.0 or 172.16.0.0
|
Routers route traffic
based on network address.
|
|
Node address of all 1s
|
Represents all hosts in a
network. Also called the broadcast address. Example 172.16.255.255 or
192.168.10.255
|
Used to send broadcasts
to all hosts in a network.
|
|
Entire address of 0s
|
Represents “any network”.
|
Used by routers to
designate the default route.
|
|
Entire IP set to all 1s.
|
Represents all hosts in
network.
|
Used to send broadcast
messages
|
|
127.0.0.1
|
Represents the loopback
address which is essentially the host itself
|
To send traffic from the
host to itself. If you want to connect to a webserver running on the host
itself, you will use this address in the browser.
|