Ipv4 tutorial

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IPV4 TUTORIAL
Simply Easy Learning by tutorialspoint.com
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Simply Easy Learning
ABOUT THE TUTORIAL
IPv4 Tutorial
Internet Protocol version 4 (IPv4) is the fourth version in the development of the Internet Protocol (IP)
and the first version of the protocol to be widely deployed. IPv4 is described in IETF publication RFC 791
(September 1981), replacing an earlier definition (RFC 760, January 1980).
This tutorial will help you in understanding IPv4 and associated terminologies along with appropriate
references and examples.
Audience
This tutorial has been designed to help beginners understand basic concepts of IPv4 required to work
with any TCP/IP based protocols. After completing this tutorial you will find yourself at a moderate level
of expertise of IPv4 from where you can take yourself to next levels.
Prerequisites
Before you start proceeding with this tutorial, I'm making an assumption that you are already aware
about basic computer and network concepts like what is a protocol, why do we need protocol, Network
Layers etc.
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Table of Content
IPv4 Tutorial ............................................................................... 2
Audience..................................................................................... 2
Prerequisites .............................................................................. 2
Copyright & Disclaimer Notice ................................................... 2
IPv4 Overview ............................................................................ 5
What is Network? .......................................................................................5
Host Addressing.........................................................................................6
IPv4 OSI Model .......................................................................... 7
Network Layer............................................................................................8
IPv4 TCP/IP Model..................................................................... 9
Internet Protocol Version 4 (IPv4) ............................................................10
IPv4 Packet Structure .............................................................. 11
IPv4 Addressing ....................................................................... 13
Unicast Addressing Mode: .......................................................................13
Broadcast Addressing Mode: ...................................................................14
Multicast Addressing Mode: .....................................................................14
Hierarchical Addressing Scheme .............................................................15
Subnet Mask ............................................................................................15
Binary Representation..............................................................................16
IPv4 Address Classes .............................................................. 18
Class A Address.......................................................................................18
Class B Address.......................................................................................19
Class C Address ......................................................................................19
Class D Address ......................................................................................19
Class E Address.......................................................................................19
IPv4 Subnetting (CIDR)............................................................ 20
Class A Subnets.......................................................................................20
Class B Subnets.......................................................................................21
Class C Subnets ......................................................................................22
IPv4 VLSM ............................................................................... 23
Step - 1.....................................................................................................23
Step - 2.....................................................................................................23
Step - 3.....................................................................................................24
Step - 4.....................................................................................................24
Step - 5.....................................................................................................24
Step - 6.....................................................................................................24
IPv4 Reserved Addresses........................................................ 25

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Private IP Addresses................................................................................25
Loopback IP Addresses ...........................................................................25
Link-local Addresses................................................................................26
IPv4 Example ........................................................................... 27
Packet flow in network..............................................................................27
Step: 1 – Acquiring an IP Address (DHCP)..............................................28
Step: 2 – DNS query ................................................................................28
Step: 3 – ARP request..............................................................................28
IPv4 Summary.......................................................................... 29
Internet Protocol v6 (IPv6)........................................................................29

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IPv4 Overview
This era is said to be the era of computers. Computers have significantly changed lives and the way we
used to live. A computing device when connected to other computing device(s) enables us to share data and
information at lightning fast speed.
What is Network?
A Network in the world of computers is said to be a collection of interconnected hosts, via some shared media
which can be wired or wireless. A computer network enables its hosts to share and exchange data and
information over the media. Network can be a Local Area Network spanned across an office or Metro Area
Network spanned across a city or Wide Area Network which can be spanned across cities and provinces.
Computer network can be as simple as two PCs connected together via a single copper cable or it can be grown
up to the complexity where every computer in this world is connected to every other, the Internet. A network then
includes more and more components to reach its ultimate goal of data exchange. Below is a brief description of
the components involved in computer network:
 Hosts - Hosts are said to be situated at ultimate end of the network, i.e. a host is a source of information and
another host will be the destination. Information flows end to end between hosts. A host can be a user’s PC,
an internet Server, a database server etc.
 Media - If wired, then it can be copper cable, fiber optic cable, coaxial cable or if wireless, it can be free-to-air
radio frequency or some special wireless band. Wireless frequencies can be used to interconnect remote
sites too.
 Hub - A hub is a multiport repeater and it is used to connect hosts in a LAN segment. Because of low
throughputs hubs are now rarely used. Hub works on Layer-1 (Physical Layer) of OSI Model.
 Switch - A Switch is a multiport bridge and is used to connect hosts in a LAN segment. Switches are much
faster than Hubs and operate on wire speed. Switch works on Layer-2 (Data Link Layer) but Layer-3
(Network Layer) switches are also available.
 Router - A router is Layer-3 (Network Layer) device which makes routing decisions for the data/information
sent for some remote destination. Routers make the core of any interconnected network and the Internet.
 Gateways - A software or combination of software and hardware putting together works for exchanging data
among networks which are using different protocols for sharing data.
 Firewall - Software or combination of software and hardware, used to protect users’ data from unintended
recipients on the network/internet.
All components in a network ultimately serve the hosts.
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Host Addressing
Communication between hosts can happen only if they can identify each other on the network. In a single
collision domain (where every packet sent on the segment by one host is heard by every other host) hosts can
communicate directly via MAC address.
MAC address is a factory coded 48-bits hardware address which can also uniquely identify a host in the world.
But if a host wants to communicate with a remote host, i.e. not in the same segment or logically not connected,
then some means of addressing is required to identify the remote host uniquely. A logical address is given to all
hosts connected to Internet and this logical address is calledInternet Protocol Address.

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IPv4 OSI Model
International Standard Organization has a well-defined Model for Communication Systems known as Open
System Interconnection, or OSI Model. This layered model is a conceptualized view of how one system should
communicate with the other, using various protocols defined in each layer. Further, each layer is designated to a
well-defined part of communication system. For example, the Physical layer defines all the components of
physical nature, i.e. wires, frequencies, pulse codes, voltage transmission etc. of a communication system.
OSI Model has following seven layers:
 Application Layer (Layer-7): This is where the user application sits who needs to transfer data between or
among hosts. For example: HTTP, file transfer application (FTP) and electronic mail etc.
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 Presentation Layer (Layer-6): This layer helps to understand data representation in one form on a host to
other host in their native representation. Data from the sender is converted to on-the-wire data (general
standard format) and at the receiver’s end it is converted to the native representation of the receiver.
 Session Layer (Layer-5): This layer provides session management capabilities between hosts. For example
if some host needs a password verification for access and if credentials are provided then for that session
password verification does not happen again. This layer can assist in synchronization, dialog control and
critical operation management (e.g., an online bank transaction).
 Transport Layer (Layer-4): This layer provides end to end data delivery between/among hosts. This layer
takes data from above layer and breaks it into smaller units called Segments and then gives it to Network
layer for transmission.
 Network Layer (Layer-3): This layer helps to uniquely identify hosts beyond the subnets and defines the
path which the packets will follow or be routed to reach the destination.
 Data Link Layer (Layer-2): This layer takes the raw transmission data (signal, pulses etc.) from Physical
Layer and makes Data Frames and sends that to upper layer and vice versa. This layer also checks any
transmission errors and sort it out accordingly.
 Physical Layer (Layer-1): This layer deals with hardware technology and actual communication mechanism
like signaling, voltage, cable type and length etc.
Network Layer
The network layer is responsible for carrying data from one host to another. It provides means to allocate logical
addresses to hosts and identify them uniquely using the same. Network layer takes data units from Transport
Layer and cuts them in to smaller unit called Data Packet.
Network layer defines the data path, the packets should follow to reach the destination. Routers work on this layer
and provides mechanism to route data to its destination.

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IPv4 TCP/IP Model
Majorly of the internet uses a protocol suite called the Internet Protocol Suite also known as TCP/IP
protocol suite. This suite is a combination of protocols which encompasses a number of different protocols for
different purpose and need. Because the two major protocols in this suites are TCP (Transmission Control
Protocol) and IP (Internet Protocol), this is commonly termed as TCP/IP Protocol suite. This protocol suite has its
own reference model which it follows over the internet. In contrast with OSI model, this model of protocols
contains less layers.
[ Comparative depiction of OSI and TCP/IP Reference Models ]
This model is indifferent to the actual hardware implementation, i.e. Physical layer of OSI Model. This is why this
model can be implemented on almost all underlying technologies. Transport and Internet layers correspond to the
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same peer layers. All three top layers of OSI Model are compressed together in single Application layer of TCP/IP
Model.
Internet Protocol Version 4 (IPv4)
Internet Protocol is one of the major protocol in TCP/IP protocols suite. This protocol works at Network layer of
OSI model and at Internet layer of TCP/IP model. Thus this protocol has the responsibility of identification of hosts
based upon their logical addresses and to route data between/among them over the underlying network.
IP provides a mechanism to uniquely identify host by IP addressing scheme. IP uses best effort delivery, i.e. it
does not guarantee that packets would be delivered to destined host but it will do its best to reach the destination.
Internet Protocol version 4 uses 32-bit logical address.

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IPv4 Packet Structure
Internet Protocol being a layer-3 protocol (OSI) takes data Segments from layer-4 (Transport) and divides it
into what’s called packet. IP packet encapsulates data unit received from above layer and adds its own header
information.
The encapsulated data is referred to as IP Payload. IP header contains all the necessary information to deliver
the packet at the other end.
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IP header includes many relevant information including Version Number, which, in this context, is 4. Other details
are as follows:
 Version: Version no. of Internet Protocol used (e.g. IPv4)
 IHL: Internet Header Length, Length of entire IP header
 DSCP: Differentiated Services Code Point, This is Type of Service.
 ECN: Explicit Congestion Notification, carries information about the congestion seen in the route.
 Total Length: Length of entire IP Packet (including IP header and IP Payload)
 Identification: If IP packet is fragmented during the transmission, all the fragments contain same
identification no. to identify original IP packet they belong to.
 Flags: As required by the network resources, if IP Packet is too large to handle these ‘flags’ tell that if they
can be fragmented or not. In this 3-bit flag, the MSB is always set to ‘0’.
 Fragment Offset: This offset tells the exact position of the fragment in the original IP Packet.
 Time to Live: To avoid looping in the network, every packet is sent with some TTL value set, which tells the
network how many routers (hops) this packet can cross. At each hop, its value is decremented by one and
when the value reaches zero, the packet is discarded.
 Protocol: Tells the Network layer at the destination host, to which Protocol this packet belongs to, i.e. the
next level Protocol. For example protocol number of ICMP is 1, TCP is 6 and UDP is 17.
 Header Checksum: This field is used to keep checksum value of entire header which is then used to check if
the packet is received error-free.
 Source Address: 32-bit address of the Sender (or source) of the packet.
 Destination Address: 32-bit address of the Receiver (or destination) of the packet.
 Options: This is optional field, which is used if the value of IHL is greater than 5. These options may contain
values for options such as Security, Record Route, Time Stamp etc.

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IPv4 Addressing
IPv4 supports three different type of addressing modes:
Unicast Addressing Mode:
In this mode, data is sent only to one destined host. The Destination Address field contains 32- bit IP address of
the destination host. Here client sends data to the targeted server:
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Broadcast Addressing Mode:
In this mode the packet is addressed to all hosts in a network segment. The Destination Address field contains
special broadcast address i.e. 255.255.255.255. When a host sees this packet on the network, it is bound to
process it. Here client sends packet, which is entertained by all the Servers:
Multicast Addressing Mode:
This mode is a mix of previous two modes, i.e. the packet sent is neither destined to a single host nor all the host
on the segment. In this packet, the Destination Address contains special address which starts with 224.x.x.x and
can be entertained by more than one host.

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Here a server sends packets which are entertained by more than one Servers. Every network has one IP address
reserved for network number which represents the network and one IP address reserved for Broadcast Address,
which represents all the host in that network.
Hierarchical Addressing Scheme
IPv4 uses hierarchical addressing scheme. An IP address which is 32-bits in length, is divided into two or three
parts as depicted:
A single IP address can contain information about the network and its sub-network and ultimately the host. This
scheme enables IP Address to be hierarchical where a network can have many sub-networks which in turn can
have many hosts.
Subnet Mask
The 32-bit IP address contains information about the host and its network. It is very necessary to distinguish the
both. For this, routers use Subnet Mask, which is as long as the size of the network address in the IP address.
Subnet Mask is also 32 bits long. If the IP address in binary is ANDed with its Subnet Mask, the result yields the
Network address. For example, say the IP Address 192.168.1.152 and the Subnet Mask is 255.255.255.0 then

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This way Subnet Mast helps extract Network ID and Host from an IP Address. It can be identified now that
192.168.1.0 is the Network number and 192.168.1.152 is the host on that network.
Binary Representation
The positional value method is the simplest form of converting binary from decimal value. IP address is 32 bit
value which is divided into 4 octets. A binary octet contains 8 bits and the value of each bit can be determined by
the position of bit value '1' in the octet.
Positional value of bits is determined by 2 raised to power (position – 1), that is the value of a bit 1 at position 6 is
26-1 that is 25 that is 32. The total value of the octet is determined by adding up the positional value of bits. The
value of 11000000 is 128+64 = 192. Some Examples are shown in the table below:

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IPv4 Address Classes
Internet Protocol hierarchy contains several classes of IP Addresses to be used efficiently in various situation
as per the requirement of hosts per network. Broadly, IPv4 Addressing system is divided into 5 classes of IP
Addresses. All the 5 classes are identified by the first octet of IP Address.
Internet Corporation for Assigned Names and Numbers - responsible for assigning IP addresses.
The first octet referred here is the left most of all. The octets numbered as follows depicting dotted decimal
notation of IP Address:
Number of networks and number of hosts per class can be derived by this formula:
When calculating hosts IP addresses, 2 IP addresses are decreased because they cannot be assigned to hosts
i.e. the first IP of a network is network number and the last IP is reserved for Broadcast IP.
Class A Address
The first bit of the first octet is always set to 0 (zero). Thus the first octet ranges from 1 – 127, i.e.
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Class A addresses only include IP starting from 1.x.x.x to 126.x.x.x only. The IP range 127.x.x.x is reserved for
loopback IP addresses.
The default subnet mask for Class A IP address is 255.0.0.0 which implies that Class A addressing can have 126
networks (2
7
-2) and 16777214 hosts (2
24
-2).
Class A IP address format thus, is 0NNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH
Class B Address
An IP address which belongs to class B has the first two bits in the first octet set to 10, i.e.
Class B IP Addresses range from 128.0.x.x to 191.255.x.x. The default subnet mask for Class B is 255.255.x.x.
Class B has 16384 (2
14
) Network addresses and 65534 (2
16
-2) Host addresses.
Class B IP address format is, 10NNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH
Class C Address
The first octet of Class C IP address has its first 3 bits set to 110, that is
Class C IP addresses range from 192.0.0.x to 192.255.255.x. The default subnet mask for Class B is
255.255.255.x.
Class C gives 2097152 (2
21
) Network addresses and 254 (2
8
-2) Host addresses.
Class C IP address format is 110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH
Class D Address
Very first four bits of the first octet in Class D IP addresses are set to 1110, giving a range of
Class D has IP address rage from 224.0.0.0 to 239.255.255.255. Class D is reserved for Multicasting. In
multicasting data is not destined for a particular host, that's why there is no need to extract host address from the
IP address, and Class D does not have any subnet mask.
Class E Address
This IP Class is reserved for experimental purposes only like for R&D or Study. IP addresses in this class ranges
from 240.0.0.0 to 255.255.255.254. Like Class D, this class too is not equipped with any subnet mask.

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IPv4 Subnetting (CIDR)
Each IP class is equipped with its own default subnet mask which bounds that IP class to have prefixed
number of Networks and prefixed number of Hosts per network. Classful IP addressing does not provide any
flexibility of having less number of Hosts per Network or more Networks per IP Class.
CIDR or Classless Inter Domain Routing provides the flexibility of borrowing bits of Host part of the IP address
and using them as Network in Network, called Subnet. By using subnetting, one single Class A IP addresses can
be used to have smaller sub-networks which provides better network management capabilities.
Class A Subnets
In Class A, only the first octet is used as Network identifier and rest of three octets are used to be assigned to
Hosts (i.e. 16777214 Hosts per Network). To make more subnet in Class A, bits from Host part are borrowed and
the subnet mask is changed accordingly.
For example, if one MSB (Most Significant Bit) is borrowed from host bits of second octet and added to Network
address, it creates two Subnets (2
1
=2) with (2
23
-2) 8388606 Hosts per Subnet.
The Subnet mask is changed accordingly to reflect subnetting. Given below is a list of all possible combination of
Class A subnets:
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In case of subnetting too, the very first and last IP address of every subnet is used for Subnet Number and
Subnet Broadcast IP address respectively. Because these two IP addresses cannot be assigned to hosts, Sub-
netting cannot be implemented by using more than 30 bits as Network Bits which provides less than two hosts per
subnet.
Class B Subnets
By Default, using Classful Networking, 14 bits are used as Network bits providing (2
14
) 16384 Networks and (2
16
-
1) 65534 Hosts. Class B IP Addresses can be subnetted the same way as Class A addresses, by borrowing bits
from Host bits. Below is given all possible combination of Class B subnetting:

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Class C Subnets
Class C IP addresses normally assigned to a very small size network because it only can have 254 hosts in a
network. Given below is a list of all possible combination of subnetted Class B IP address:

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IPv4 VLSM
Internet Service Providers may face a situation where they need to allocate IP subnets of different sizes as
per the requirement of customer. One customer may ask Class C subnet of 3 IP addresses and another may ask
for 10 IPs. For an ISP, it is not feasible to divide the IP addresses into fixed size subnets, rather he may want to
subnet the subnets in such a way which results in minimum wastage of IP addresses.
For example, an administrator have 192.168.1.0/24 network. The suffix /24 (pronounced as "slash 24") tells the
number of bits used for network address. He is having three different departments with different number of hosts.
Sales department has 100 computers, Purchase department has 50 computers, Accounts has 25 computers and
Management has 5 computers. In CIDR, the subnets are of fixed size. Using the same methodology the
administrator cannot fulfill all the requirements of the network.
The following procedure shows how VLSM can be used in order to allocate department-wise IP addresses as
mentioned in the example.
Step - 1
Make a list of Subnets possible.
Step - 2
Sort the requirements of IPs in descending order (Highest to Lowest).
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 Sales 100
 Purchase 50
 Accounts 25
 Management 5
Step - 3
Allocate the highest range of IPs to the highest requirement, so let's assign 192.168.1.0 /25 (255.255.255.128) to
Sales department. This IP subnet with Network number 192.168.1.0 has 126 valid Host IP addresses which
satisfy the requirement of Sales Department. The subnet Mask used for this subnet has 10000000 as the last
octet.
Step - 4
Allocate the next highest range, so let's assign 192.168.1.128 /26 (255.255.255.192) to Purchase department.
This IP subnet with Network number 192.168.1.128 has 62 valid Host IP Addresses which can be easily assigned
to all Purchase department's PCs. The subnet mask used has 11000000 in the last octet.
Step - 5
Allocate the next highest range, i.e. Accounts. The requirement of 25 IPs can be fulfilled with 192.168.1.192 /27
(255.255.255.224) IP subnet, which contains 30 valid host IPs. The network number of Accounts department will
be 192.168.1.192. The last octet of subnet mask is 11100000.
Step - 6
Allocate next highest range to Management. The Management department contains only 5 computers. The
subnet 192.168.1.224 /29 with Mask 255.255.255.248 has exactly 6 valid host IP addresses. So this can be
assigned to Management. The last octet of subnet mask will contain 11111000.
By using VLSM, the administrator can subnet the IP subnet such a way that least number of IP addresses are
wasted. Even after assigning IPs to every department, the administrator, in this example, still left with plenty of IP
addresses which was not possible if he has used CIDR.

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IPv4 Reserved Addresses
There are few Reserved IPv4 address spaces which cannot be used on the internet. These addresses
serve special purpose and cannot be routed outside Local Area Network.
Private IP Addresses
Every class of IP, (A, B & C) has some addresses reserved as Private IP addresses. These IPs can be used
within a network, campus, company and are private to it. These addresses cannot be routed on Internet so
packets containing these private addresses are dropped by the Routers.
In order to communicate with outside world, Internet, these IP addresses must have to be translated to some
public IP addresses using NAT process or Web Proxy server can be used.
The sole purpose to create separate range of private addresses is to control assignment of already-limited IPv4
address pool. By using private address range within LAN, the requirement of IPv4 addresses has globally
decreased significantly. It has also helped delaying the IPv4 address exhaustion.
IP class, while using private address range, can be chosen as per the size and requirement of the organization.
Larger organization may choose class A private IP address range where smaller may opt for class C. These IP
addresses can be further sub-netted be assigned to departments within an organization.
Loopback IP Addresses
The IP address range 127.0.0.0 – 127.255.255.255 is reserved for loopback i.e. a Host’s self-address. Also
known as localhost address. This loopback IP address is managed entirely by and within the operating system.
Using loopback addresses, enable the Server and Client processes on a single system to communicate with each
other. When a process creates a packet with destination address as loopback address, the operating system
loops it back to itself without having any interference of NIC.
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Data sent on loopback is forward by the operating system to a virtual network interface within operating system.
This address is mostly used for testing purposes like client-server architecture on a single machine. Other than
that, if a host machine can successfully ping 127.0.0.1 or any IP from loopback range, implies that the TCP/IP
software stack on the machine is successfully loaded and working.
Link-local Addresses
In case of the Host is not able to acquire an IP address from DHCP server and it has not been assigned any IP
address manually, the host can assign itself an IP address from a range of reserved Link-local addresses. Link
local address range is 169.254.0.0 - 169.254.255.255.
Assume a network segment where all systems are configured to acquire IP addresses from a DHCP server
connected to the same network segment. If the DHCP server is not available, no host on the segment will be able
to communicate to any other. Windows (98 or later), and Mac OS (8.0 or later) support this functionality of self-
configuration of Link-local IP address. In absence of DHCP server, every host machine randomly chooses an IP
address from the above mentioned range and then checks to ascertain by means of ARP, if some other host also
has not configured himself with the same IP address. Once all hosts are using link local addresses of same
range, they can communicate to each other.
These IP addresses cannot help system to communicate when they do not belong to the same physical or logical
segment. These IPs are also not routable.

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IPv4 Example
This section tells how actual communication happens on the Network using Internet Protocol version 4.
Packet flow in network
All the hosts in IPv4 environment are assigned unique logical IP addresses. When a host wants to send some
data to another host on the network, it needs the physical (MAC) address of the destination host. To get the MAC
address, the host broadcasts ARP message and asks to give the MAC address whoever is the owner of
destination IP address. All the host on that segment receives ARP packet but only the host which has its IP
matching with the one in ARP message, replies with its MAC address. Once the sender receives the MAC
address of receiving station, data is sent on the physical media.
In case, the IP does not belong to the local subnet. The data is sent to the destination by means of Gateway of
the subnet. To understand the packet flow we must first understand following components:
 MAC Address: Media Access Control Address is 48-bit factory hard coded physical address of network
device which can uniquely be identified. This address is assigned by device manufacturers.
 Address Resolution Protocol: Address Resolution Protocol is used to acquire the MAC address of a host
whose IP address is known. ARP is a Broadcast packet which is received by all the host in the network
segment. But only the host whose IP is mentioned in ARP responds to it providing its MAC address.
 Proxy Server: To access Internet, networks use Proxy Server which has a public IP assigned. All PCs
request Proxy Server for a Server on Internet, Proxy Server on behalf of PC sends the request to server and
when it receives response from the Server, the Proxy Server forwards it to the client PC. This is a way to
control Internet access in computer networks and it helps to implement web based policies.
 Dynamic Host Control Protocol: DHCP is a service by which a host is assigned IP address from a pre-
defined address pool. DHCP server also provides necessary information such as Gateway IP, DNS Server
Address, lease assigned with the IP etc. By using DHCP services network administrator can manage
assignment of IP addresses at ease.
 Domain Name System: This is very likely that a user does not know the IP address of a remote Server he
wants to connect to. But he knows the name assigned to it for example, tutorialpoints.com. When the user
types in the name of remote server he wants to connect to the localhost behind the screens sends a DNS
query. Domain Name System is a method to acquire the IP address of the host whose Domain Name is
known.
 Network Address Translation: Almost all PCs in a computer network are assigned private IP addresses
which are not routable on Internet. As soon as a router receives an IP packet with private IP address it drops
it. In order to access Servers on public private address, computer networks use an address translation
service, which translates between public and private addresses, called Network Address Translation. When a
PC sends an IP packet out of a private network, NAT changes the private IP address with public IP address
and vice versa.
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We can now describe the packet flow. Assume that a user wants to access www.TutorialsPoint.com from her
personal computer. She is having internet connection from her ISP. The following steps will be taken by the
system to help her reach destination website.
Step: 1 – Acquiring an IP Address (DHCP)
When user’s PC boots up, it searches for a DHCP server to acquire an IP address. For the same, PC sends a
DHCPDISCOVER broadcast which is received by one or more DHCP servers on the subnet and they all respond
with DHCPOFFER which includes all the necessary details like IP, subnet, Gateway, DNS etc. PC sends
DHCPREQUEST packet in order to request the offered IP address. Finally, DHCP sends DHCPACK packet to tell
PC that it can keep the IP for some given amount of time aka IP lease.
Alternatively a PC can be assigned an IP address manually without taking any help from DHCP Server. When a
PC is well configured with IP address details, it can now speak to other computers all over the IP enabled
network.
Step: 2 – DNS query
When a user opens a web browser and types www.tutorialpoints.com which is a domain name and a PC does not
understand how to communicate with the server using domain names. PC sends a DNS query out on the network
in order to obtain the IP address pertaining to the domain name. The pre-configured DNS server responds the
query with IP address of the domain name specified.
Step: 3 – ARP request
The PC finds that the destination IP address does not belong to his own IP address range and it has to forward
the request to the Gateway. Gateway in this scenario can be a router or a Proxy Server. Though Gateway’s IP
address is known to the client machine but computers do not exchange data on IP addresses rather they need
machine’s hardware address which is Layer-2 factory coded MAC address. To obtain the MAC address of the
Gateway the client PC broadcasts an ARP request saying "Who owns this IP address?" The Gateway in response
to the ARP query sends it MAC address. Upon receiving MAC address PC sends the packets to Gateway.
An IP packet has both source and destination addresses and this connects host with a remote host logically.
Whereas MAC addresses helps systems on a single network segment to transfer actual data. This is important
that source and destination MAC addresses change as they travel across the Internet (segment by segment) but
source and destination IP address never changes.

TUTORIALS POINT
IPv4 Summary
The Internet Protocol version 4 was designed to be allocated to approx. 4.3 billion addresses. At the
beginning of Internet this was considered a much wider address space for which there was nothing to worry
about.
The sudden growth in Internet users and its wide spread use has exponentially increase the number of devices
which needs real and unique IP to be able to communicate. Gradually, an IP is required by almost every digital
equipment which were made to ease human life, such as Mobile Phones, Cars and other electronic devices. The
number of devices (other than computers/routers) expanded the demand for extra IP addresses, which were not
considered earlier.
Allocation of IPv4 is globally managed by Internet Assigned Numbers Authority (IANA) under coordination with
Internet Corporation for Assigned Names and Numbers (ICANN). IANA works closely with Regional Internet
Registries, which in turns are responsible for efficiently distribute IP address in their territories. There are five such
RIR exist. According to IANA reports, all the IPv4 address blocks have been allocated. To cope up with the
situation, as an early steps the following practices were being done:
 Private IPs: Few blocks of IPs were declared for private use within a LAN so that the requirement for public
IP addresses can be reduced.
 NAT: Network address translation is a mechanism by which multiple PCs/hosts with private IP addresses are
enabled to access using one or few public IP addresses.
 Unused Public IPs were reclaimed by RIRs.
Internet Protocol v6 (IPv6)
IETF (Internet Engineering Task Force) has redesigned IP addresses and to mitigate the drawbacks of IPv4. The
new IP address has version 6 and is 128-bit address, by which every single inch of the earth can be given millions
of IP addresses.
Today majority of devices running on Internet are using IPv4 and it is not possible to shift them to IPv6 in coming
days. There are mechanisms provided by IPv6, by which IPv4 and IPv6 can coexist unless the Internet entirely
shifts to IPv6:
 Dual IP Stack
 Tunneling (6to4 and 4to6)
 NAT Protocol Translation
CHAPTER
11

Ipv4 tutorial

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