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Computer Network
Dr.S.Santhi
Professor, Department of CSE
KIT-Kalaignarkarunanidhi Institute of Technology

Introduction
Identifier of each device connected to the Internet : IP
Address
IPv4 Address : 32 bits
The address space of IPv4 is 232 or 4,294,967,296
The IPv4 addresses are unique and universal
Two devices on the Internet can never have the same address
at same time
Number in base 2, 16, and 256
Refer to Appendix B

Binary Notation and Dotted-Decimal Notation
Binary notation
01110101 10010101 00011101 11101010
32 bit address, or a 4 octet address or a 4-byte address
Decimal point notation

Notation (cont’d)
Hexadecimal Notation
- 8 hexadecimal digits
- Used in network programming
0111 0101 1001 0101 0001 1101 1110 1010
75 95 1D EA
0x75951DEA

Example
 Change the following IPv4 addresses from binary notation to dotted-decimal
notation
a. 10000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
c. 11100111 11011011 10001011 01101111
d. 11111001 10011011 11111011 00001111
 Solution
We replace each group of 8 bits with its equivalent decimal number (see Appendix B)
and add dots for separation.
a. 129.11.11.239
b. 193.131.27.255
c. 231.219.139.111
d. 249.155.251.15

Example
Change the following IPv4 address in hexadecimal notation.
a. 10000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
Solution
We replace each group of 4 bits with its hexadecimal
equivalent. Note that hexadecimal notation normally has no
added spaces or dots; however, 0x is added at the beginning of
the subscript 16 at the end
a. 0X810B0BEF or 810B0BEF16
b. 0XC1831BFF or C1831BFF16

Example
Find the number of addresses in a range if the first
address is 146.102.29.0 and last address is
146.102.32.225.
Solution
We can subtract the first address from the last address in
base 256(see Appendix B). The result is 0.0.3.255 in this
base. To find the number of addresses in the range, we
convert this number to base 10 and add 1 to the result
Number of addresses = (0 x 2563 + 0 x 2562 + 3 x 2561 + 255 x 2560)+ 1 = 1024

Operations
Need to apply some operations on 32-bit numbers in
binary or dotted-decimal notation.
Bitwise NOT operation

Operations(con’t)
Bitwise AND operation

Operations(cont’d)
Bitwise OR operation

Classful Addressing
IP addresses, when started a few decades ago, used
the concept of classes
In the mid-1990s, a new architecture, called classless
addressing, was introduced
We will discuss classful addressing in this section,
first. Classless addressing will be discussed in next
section.

Occupation of the Address Space
Five classes

Finding the Class of an Address

Finding the Addresses Class Using Continuous Checking
1
Class: A
0
Start
1
0
Class: B
1
0
Class: C
1
0
Class: D Class: E

Example
Find the class of each address:
a. 00000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
c. 10100111 11011011 10001011 01101111
d. 11110011 10011011 11111011 00001111
Solution
See the procedure in Figure 5.7
a. The first bit is 0. This is a class A address.
b. The first 2 bits are 1; the third bit is 0. This is a class C address.
c. The first bit is 1; the second bit is 0. This is a class B address.
d. The first 4 bits are 1s. This is a class E address.

Netid and hostid of A, B, and C Classes
netid and hostid are of varying lengths, depending on
the class of the address

Blocks in Class A
Only 1 byte in class A defines the netid
The leftmost bit should be ‘0’
Class A is divided into 27 = 128 blocks
Each block in class A contains 16,777,216 addresses

Blocks in Class B
2 bytes in class B define the class
The two leftmost bits should be ‘10’
Class B is divided into 214 = 16,384 blocks
Each block in class B contains 65,536 addresses

Blocks in Class C
3 bytes in class C define the class
The three leftmost bits should be ‘110’
Class C is divided into 221 = 2,097,152 blocks
Each block in class C contains 256 addresses

The Single Block in Class D and E
Class D
Class D is designed for multicasting
Used to define one group of hosts on the Internet
Class E
Reserved for future purposes

Information Extraction in Classful Addressing
The number of addresses
The first address
The last address
netid
First address
000 ... 0

Example
An address in a block is given as 173.22.17.25. Find the
number of addresses in the block, the first address, and the
last address
Solution
1. The number of addresses in this block is
N = 232-n = 216
2. To find the first address, we keep the left most 16 bits and
set the rightmost 16 bits all to 0s. The first address is
173.22.0.0/16 in which 16 is the value of n.
3. To find the last address, we keep the leftmost 16 bits and
set the rightmost 16 bits all to 1s. The last address is
173.22.255.255

Network Address
The first address of block is network address
Used in routing a packet to its destination network
The network address is the identifier of a network

Network Mask
Used to extract the network address from the
destination address of a packet
Called a default mask

Finding a Network Address using the Default Mask

Example
A router receives a packet with the destination address
201.24.67.32. Show how the router finds the network
address of the packet.
Solution
Since the class of the address is B, we assume that the
router applies the default mask for class B, 255.255.0.0 to
find the network address.
Destination address -> 201 . 24 . 67 . 32
Default mask -> 255 . 255 . 0 . 0
Network address -> 201 . 24 . 0 . 0

Three-Level Addressing : Subnetting
The organization that was granted a block in class A or
B needed to divide its large network into several
subnetworks for better security and management
In subnetting, a network is divided into several smaller
subnetworks with each subnetwork having its own
subnetwork address

Example
 The next figure shows a network using class B addresses before
subnetting. We have just one network with almost 216 hosts. The
whole network is connected, through one single connection, to
one of the routers in the Internet. Note that we have shown /16 to
show the length of the netid (class B)

Example
The next figure shows same network in example 5.18
after subnetting.

Supernetting
Combine several class C blocks to create a larger range
of address
An organization that needs 1000 addresses can be
granted four class C blocks.
Supernet mask is the reverse of a subnet mask

Comparison of Subnet, Default, and Supernet masks

Classless Addressing
Classful address did not solve the address depletion
problem
Distribution of addresses and the routing process more difficult
With the growth of the Internet, a larger address space was
needed as a long-term solution
Although the long-range solution has already been devised
and is called IPv6, a short-term solution was also devised to
use the same address space but to change the distribution
of addresses
Classless addressing

Variable-length blocks in Classless Addressing
In classless addressing, whole address space id
divided into variable length blocks
Theoretically, we can have a block of 20, 21, 22, … 232
addresses

Prefix and Suffix
Prefix : play the same role as the netid
Suffix : play the same role as the hostid
The prefix length in classless addressing can be 1 to 32

Example
What is the prefix length and suffix length if the whole
Internet is considered as one single block with
4,294,967,296 addresses?
Solution
In this case, the prefix length is 0 and suffix length is 32.
All 32 bits vary to define 232 = 4,294,967,296 hosts in this
single block

Slash Notation
Notation of address including length of prefix
In classless addressing, we need to know one of the
addresses in the block and the prefix length to define
the block

Example
In classless addressing, an address cannot per se
define the block the address belongs to. For example,
the address 230.8.24.56 can belong to many blocks
some of them are shown below with the value of the
prefix associated with that block :

Example
One of the address in a block is 167.199.170.82/27. To find the
number of addresses in the network, the first address, and the last
address.
Solution
The value of n is 27. The network mask has twenty-seven 1s and five
0s. It is 255.255.255.240.
a.The number of addresses in the network is 232-n = 25 = 32
b.We use the AND operation to find the first address. The first address is
167.199.170.64/27
Address in Binary 10100111 11000111 10101010 01010010
Network mask 11111111 11111111 11111111 11100000.
First address 10100111 11000111 10101010 01000000

Example (cont’d)
c. To find the last address, we first find the complement of the network mask
and the OR it with the given address : the last address is 167.199.170.95/27
Address in Binary 10100111 11000111 10101010 01010010
Network mask 0000000 00000000 00000000 00011111
Last address 10100111 11000111 10101010 01011111

Extracting Block Information
The number of addresses in the block can be found as
N = 232-n
The first address in the block can be found by ANDing
the address with the network mask
First address = (any address) AND (network mask)
The last address in the block can be found by either
adding the first address with the number of addresses
or, directly, by ORing the address with complement
(NOTing) of the network mask
Last address = (any address) OR [NOT (network mask)]

Block Allocation
The ultimate responsibility of block allocation is given to a
global authority called ICANN(Internet Corporation for
Assigned Names and Address)
Assign a large block of addresses to an ISP (Internet Service
Provider)
For the proper operation of the CIDR, there are three
restrictions
The number of requested addresses, N, needs to be power of 2.
The value of prefix length can be found from the number of
addresses in the block
The requested block needs to be allocated where there are a
contiguous number of unallocated addresses in the address
space

Example
An ISP has requested a block of 1000 addresses. The
following block is granted.
a.Since 1000 is not a power of 2, 1024 addresses are
granted
b.The prefix length for the block is calculated as n = 32 –
log21024 = 22
c.The beginning address is chosen as 18.14.12.0
The granted block is 18.14.12.0/22. The first address is
18.14.12.0/22 and the last address is 18.14.15.255/22

Special Addresses
In classful addressing some addresses were reserved for
special purposes. The classless addressing scheme inherits
some of these special addresses from classful addressing
Special block
All-Zero Address
All-One Address
Loopback Address
Private Address
Multicast Address
Special address in each block
Network Address
Direct broadcast address

Example of using the all-zeros address
When a host needs to send an IPv4 packet but it does
not know its own address
Source: 0.0.0.0
Destination: 255.255.255.255
Packet

Example of Limited Broadcast Address
All-One Address
A host that wants to send a message to every other
host can use
221.45.71.20/24 221.45.71.178/24
221.45.71.64/24 221.45.71.126/24
Network

Example of Loopback Address
Used to test the software on a machine

Private address
Not recognize globally
Used either in isolation or in connection with network
address translation technique
Block Number of addresses Block Number of address
10.0.0.0/8 16,777,216 192.168.0.0/16 65,536
172.16.0.0/12 1,047,584 169.254.0.0/16 65,536

Example of Directed Broadcast Address

NAT
The distribution of addresses through ISPs has created a
new problem
ISP cannot rearrange the range of addresses
But in most situation, only a portion of computers in a small
network need access to the Internet simultaneously
The number of allocated addresses does not have to match the
number of computers in the network
A technology that can provide the mapping between the
private and universal addresses, and at the same time,
support virtual private networks is NAT(Network Address
Translation)

Example of NAT and Address Translation

Example of NAT
Using a pool of IP address
Using one global address by the NAT router allows only
one private-network host to access the same external
host
 To remove this restriction, NAT router can use a pool of
global addresses
Using both IP addresses and port addresses
To allow a many-to-many relationship
Private Address Private Port External Address External Port Transport Protocol
172.18.3.1 1400 25.8.3.2 80 TCP
172.18.3.2 1401 25.8.3.2 80 TCP
… … … … ...

Summary (1)
 The identifier used in the IP layer of the TCP/IP layer of the TCP/IP protocol suite is called the
Internet address or IP address. An IPv4 address is 32 bits long. An address space is the total
number of addresses used by the protocol. The address space of IPv4 is 232 or 4,294,967,296.
 In classful addressing, the IPv4 address space is divided into five classes: A, B, C, D, and E.
An organization is granted a block in one of the three classes, A, B, or C. Classes D and E is
reserved for special purpose. An IP address in classes A, B, and C is divided into netid and
hostid.
 In classful addressing, the first address in the block is called the network address. It defines
the network to which an address belongs. The network address is used in routing a packet to
its destination network.
 A network mask or a default mask in classful addressing is a 32-bit number with n leftmost
bits all set to 1s and (32 – n) rightmost bits all set to 0s. It is used by a router to find the
network address from the destination address of a packet.
 The idea of splitting a network into smaller subnetworks is called subnetting. A subnetwork
mask, like a network mask, is used to find the subnetwork address when a destination IP
address is given. In supernetting, an organization can combine several class C blocks to
create a larger range of addresses.

Summary (2)
 In 1996, the Internet authorities announced a new architecture called classless
addressing, or CIDR (classless Inter-domain routing) that allows an organization to
have a block addresses of any size as long as the size of the block is a power of two.
 The address in classless addressing is also divided into two parts: the prefix and the
suffix. The prefix plays the same role as the netid; the suffix plays the same role as
the hostid. All addresses in the block have the same prefix; each address has a
different suffix
 Some of the blocks in IPv4 are reserved for special purpose. In addition, some
addresses in a block are traditionally used for special addresses. These addresses
are not assigned to any host.
 To improve the distribution of addresses, NAT technology has been created to allow
separation of private addresses in a network from the global addresses used in the
Internet. A translation table can translate the private addresses, selected from the
blocks allocated for this purpose, to global addresses. The translation table also
translates the IP addresses as well as the port number for mapping from the private
to global addresses and vice versa.

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