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So an IP Address consists of 2 main portions

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we have the Network Address Portion also known as the Network ID

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this identifies a specific network, routers maintain routing

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tables that contain Network Addresses it's important to realize

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that routers build their routing tables based on the Network

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Address and not on the host address, so they do not route
packets

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from 1 interface to another interface based on IP Addresses

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they do their routing based on Network Addresses, so they will

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look at the destination IP Address in a packet and match that

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00:00:39,000 --> 00:00:43,000
to a Network Address in their routing table to determine how

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traffic is routed. So an IP Address consist of the Network

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portion as well as the host portion which is also called

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the Host ID, this identifies specific end point on a Network

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such as a server, a printer, a PC, an iPhone, an iPad or some

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other type of device including IP Phones, these are essentially

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devices that need to communicate with each other, so PC may be

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communicating with the server, you may be using your iPad to
read

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the news, so you'll be surfing to cnn.com or usatoday.com

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or the BBC, so your iPad is sending traffic to that server and

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the server is returning traffic to your iPad, so the iPad and

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the server will have a TCP session established at Layer 4 but
for

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the iPad to communicate with the server, routers need to route

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traffic to the server and back again

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So here's an analogy to explain Network and host

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in cities throughout the world you have streets, so as an

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example we have Oxford St., on the street we have multiple
houses

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or multiple offices but in my example let's assume we have
houses

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on the street, each house has an individual house number

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so we have in this example 1 Oxford St. 2 Oxford St.

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3 Oxford St., 4 Oxford St., 5 Oxford St. and so forth and so on

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each house number uniquely identifies that specific house on

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this street, now you wouldn't have 2 houses with the same number

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on the same street, that would be very confusing because perhaps

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you want to meet your friends, the Joneses at #1 Oxford St.

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but you may end up spending your day with the totally

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different group of people if you have 2 houses with the same

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number, in this example we have 2 #1 Oxford St. houses

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Which house is the correct house?

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Now that's not desirable, when you go to 1 Oxford St. you want

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to make sure that you are going to the right house to meet

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your friends, rather than going to some other house and hence

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house numbers need to be unique on a street

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00:03:00,000 --> 00:03:05,000
in a City, in the World. Now continuing with our analogy

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00:03:05,000 --> 00:03:08,000
when working out how to get to remote destination

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in the physical world, let's say you want to go to Oxford St.

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00:03:12,000 --> 00:03:16,000
in London, in the UK and you've never been before

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00:03:16,000 --> 00:03:20,000
you may use a mapping technologies such as Google Maps to find

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00:03:20,000 --> 00:03:24,000
out how to get to that specific street, now when you're doing

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00:03:24,000 --> 00:03:28,000
this you don't look at individual house numbers but rather look

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00:03:28,000 --> 00:03:32,000
at street names to determine your way to get to that destination

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00:03:32,000 --> 00:03:36,000
So if you're in different part of London, you may use the street

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00:03:36,000 --> 00:03:41,000
name to work out how to get from you current street to Oxford
St.

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00:03:41,000 --> 00:03:46,000
and when you get to the destination street then you're able to

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00:03:46,000 --> 00:03:50,000
look at house numbers to determine which house number to go to

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00:03:50,000 --> 00:03:53,000
technologies such as Google Maps are obviously really
intelligent

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and know the house numbers and will guide you to the right

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part of the street, but for this analogy thinks of how routers

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00:04:01,000 --> 00:04:06,000
route, they will route traffic to a destination Network and when

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00:04:06,000 --> 00:04:11,000
traffic gets to that destination Network then house number are

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00:04:11,000 --> 00:04:15,000
used or IP Addresses are used to determine which host

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00:04:15,000 --> 00:04:16,000
to forward the traffic to

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00:04:16,000 --> 00:04:22,000
Now in a similar way, the Network address in this case 10.1.1.0

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00:04:22,000 --> 00:04:26,000
identifies a specific network somewhere in the World

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00:04:26,000 --> 00:04:31,000
Host Addresses such as 1 in this example identify individual

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00:04:31,000 --> 00:04:36,000
devices on that network, so 1 identifies this PC

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00:04:36,000 --> 00:04:41,000
on Network 10.1.1.0, each device

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00:04:41,000 --> 00:04:45,000
on this network has a unique number, so in this example we have

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00:04:45,000 --> 00:04:49,000
5 unique devices in the same way that in our previous analogy

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00:04:49,000 --> 00:04:53,000
we had 5 houses on Oxford St. now in the same way

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00:04:53,000 --> 00:04:58,000
you wouldn't configure multiple PCs or multiple devices with

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00:04:58,000 --> 00:05:02,000
the same IP Address as that would cause confusion and conflicts

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00:05:02,000 --> 00:05:06,000
So as an example, if another device wants to communicate with

73
00:05:06,000 --> 00:05:10,000
10.1.1.1 which PC would it send the traffic to?

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00:05:10,000 --> 00:05:16,000
If PC 5 wants to communicate with this host 10.1.1.1

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00:05:16,000 --> 00:05:21,000
there are 2 hosts with the same IP Address which device should

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00:05:21,000 --> 00:05:25,000
PC 5 communicates with? So devices on a Network or subnet

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00:05:25,000 --> 00:05:29,000
which we'll talk more about in a moment have a unique IP Address

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00:05:29,000 --> 00:05:33,000
in the same way that a house has a unique house number

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00:05:33,000 --> 00:05:37,000
on a specific street, so in the same way

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00:05:37,000 --> 00:05:40,000
that we use street names when going from A to B

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00:05:40,000 --> 00:05:45,000
routers look at Network Addresses to determine how to get

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00:05:45,000 --> 00:05:49,000
to a remote destination, other protocols such as

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00:05:49,000 --> 00:05:53,000
Address Resolution Protocol or ARP are used to find

84
00:05:53,000 --> 00:05:57,000
the house number or host number for a host on a specific street

85
00:05:57,000 --> 00:06:01,000
or network, so in this analogy we're going from Queen Anne St.

86
00:06:01,000 --> 00:06:06,000
to Oxford St. and we would use street names for our directions

87
00:06:06,000 --> 00:06:11,000
as per this example, but once we get to this street in this case

88
00:06:11,000 --> 00:06:14,000
Oxford St. we would look for the specific house by looking at

89
00:06:14,000 --> 00:06:18,000
house numbers in the physical world or in the Network world

90
00:06:18,000 --> 00:06:23,000
we would use a protocol such as ARP to find the specific host

91
00:06:23,000 --> 00:06:27,000
on that Network, routers do not make routing decisions

92
00:06:27,000 --> 00:06:32,000
based on IP Addresses and make it based on Network Addresses

93
00:06:32,000 --> 00:06:36,000
A routers routing table is not populated with IP Addresses

94
00:06:36,000 --> 00:06:40,000
it's populated with Network Addresses, the destination

95
00:06:40,000 --> 00:06:44,000
IP Address in a packet is then matched to the routing table

96
00:06:44,000 --> 00:06:47,000
populated with Network Addresses and then a determination

97
00:06:47,000 --> 00:06:52,000
is made to decide out of which port or interface on

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00:06:52,000 --> 00:06:57,000
the router the packet should be forwarded