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First of all let us understand IP? IP stands for Internet Protocol. It is a part of TCP/IP stack. IP is a layer 3 protocol according to OSI reference model. IP is mainly responsible for routing. The IP protocol uses specific address called IP address or logical address. Whenever you send any data to another system using any network based application, like internet explorer, FTP client or outlook express etc., the data travels from your network application to the TCP. TCP adds the relevant information to the data and hands it over to the IP. =================================================
The IP protocol here adds the source and destination IP address. Now let us discuss about IP address in detail.
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As
you can see in the illustration, IP address is a 32 bit binary number.
But for the ease of human reference it is being represented into dotted
decimal notations.Actually it is the dotted decimal notation which is
always assigned to the systems by network administrator. Then why do we
need to understand the binary form? Definitely this question should
arise in your mind. Well as a Cisco Certified Network Associate, you may
be required to perform complex tasks related to networking. One of the
tasks may be to subnet the IP address. I know now you will be thinking
about subnetting. Well here all I can tell you is that subnetting is a
process of dividing one network into multiple smaller networks. Let us
get back to our core topic. That is, what is IP address? As we discussed
earlier, it's a layer three or logical address used by IP protocol to
determine whereabouts of the destination system and the exact system
itself. Let us delve into further details.
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As
I told you that IP address is a 32 bit binary numbers, which identifies
to which logical group the computer belongs to and the exact host. In
other words, IP address constitutes of two parts. One part of the
address identifies the logical group or network ID of the computer and
the other part represents the host itself.Here in the illustration, up
to "192.168.0" is network portion and "1" is the host portion of the
address. In other words the computer which is assigned with an IP
address of 192.168.0.1 belongs to 192.168.0.0 network and its unique
identification is "1". Now the important question is how it is decided
that how much portion of the IP address is network address and how much
is the host ID? If you look at the binary format of the example IP
address, you will find the answer. All continuous "1's" in the subnet
mask decides the network portion of the IP address. The portion of IP
address above all continuous ones in subnet mask represents network
address. The portion of IP address above all remaining zeroes represents
host ID.
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Here
you can see that PC-1 and PC-2 are in 192.168.0.0 network. Since both
of the computers are sharing the common network id, i.e., 192.168.0, we
can call them to be belonging to same logical group. The computers
belonging to same logical group can communicate with each other directly
without the help of any intermediate device like routers. At the other
side PC-3 and PC-4 are in other logical group which is 192.168.1.0. Here
PC-3 and PC-4 both share the same network portion, i.e., 192.168.1 and
there host ID's are unique in their network. Remember, in the world of
networking, it is not the physical layout which makes different
networks, but it is the layer three addresses or logical address which
divides computers into different networks. In the illustration if we
assign all of the four computers the same network id, i.e., 192.168.0.0
then all of them will become a single network and we will not require a
device like router in between. Computers in the same network can
communicate with each other without any third device. But computers with
different network id must have some type of router in between to act as
gateway for the computers of the two networks.
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Suppose
here in the diagram if PC-1 wants to send a data packet to PC-2, what
will happen? At PC-1 the IP protocol will put its own IP address as
source address, PC-2's IP address as destination IP address in the data
packet. While a source and destination layer three addresses are being
added to the data packet by say IP protocol, the IP protocol decides
whether the packet is destined for the same network or to other network
than its own. If the destination network is same then it knows that
there is no requirement of any gateway address. It will simply send an
ARP broadcast to its own network, asking for the MAC address of the
destination machine i.e., PC-2. ARP stands for Address Resolution
Protocol and it is a part of the TCP/IP protocol stack. ARP is used to
resolve the MAC address from the known IP address. Here in our case the
layer three components know what the destination IP address is. But it
does not know the MAC address of the computer who's IP address is
192.168.0.2. So in order to know the destination machines MAC address
without which data cannot be moved out of the machine, PC-1 sends ARP
broadcast to its own network, asking for the MAC address of the PC-2.
The broadcast means request destined for all computers.
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Here
in our case the ARP broadcast will be looking something like this. You
can see that PC-1 is sending an ARP broadcast to entire network. Entire
network here is specified by 255 which is the maximum value for the host
portion. Just have a look over the Destination IP address
192.168.0.255. i.e., the packet is destined for the every computer whose
Network ID is 192.168.0. This broadcast packet will reach to every
computer at the routers E0 side. But the router will not allow this
broadcast to be propagated to other side of the router. The ARP
broadcast contains a request asking for PC-2's MAC address. In the last
line you can see that PC-2 is responding with its MAC address. This is
how computers learn about destination computers MAC address. Then layer
two protocols like Ethernet put this address as destination MAC address
before data is finally being sent out of the computer. So this whole
process was related to the communication between computers in same
network. What will happen if PC-1 wanted to communicate with PC-3?
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In
the case of PC-1 wanted to communicate with PC-3, the Layer three
protocols will discover that the destination computer is in other
network and it can't get MAC address of the destination computer
directly. Because a computer cannot broadcast any thing other than its
own network. And when the router will receive a broadcast destined for
192.168.0.0 network, it will simply drop it. So once PC-1 decided that
the destination is in other network, it knows there must be some kind of
router in between, and instead of asking for the MAC address of the
destination Machine in the ARP broadcast, it should ask for the MAC
address of the router. So now in this case PC-1 will send a broadcast to
its own network, but instead of asking for the MAC address of the
destination computer, it is asking who is having 192.168.0.3? which is
the routers address to which side the PC-1's network is connected. The
router is replying with its own MAC address to the PC-1. Once PC-1
obtains the PC-3's MAC address, it will send the packet on the network.
Now this packet will be received by the router, Since the packet's
destination MAC address will match with the router's MAC address. Now
router will check the received packets destination layer three network
ID and it will find that the destination network is directly connected
to it on Port E1. So router will send an ARP broadcast on E1 destined
for the network 192.168.1.255 asking for the MAC address of 192.168.1.1.
PC-3 will send its MAC address to the router and router will add that
MAC address to the packets destination layer two address field and
forward it to the E1 interface. So you would have understood how layer
three protocols like IP are used to divide computers into different
logical groups. You also understood that routers do not forward
broadcasts to other side. Actually we will discuss routing in later
chapters. Let us get back to the current topic that is IP addressing.
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First
of all let us get familiar with little of background. The TCP/IP
protocols were initially developed as part of the research network
developed by the United States Defense Advanced Research Projects Agency
(DARPA or ARPA) in the 1970s by pioneering network engineers Vinton
Cerf and Bob Kahn. The Designers of the IP protocols created 5 classes
of the IP addresses. Namely class A, class B, class C, class D and class
E.
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