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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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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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in a City, in the World. Now continuing with our analogy
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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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in London, in the UK and you've never been before
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you may use a mapping technologies such as Google Maps to find
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out how to get to that specific street, now when you're doing
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this you don't look at individual house numbers but rather look
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at street names to determine your way to get to that destination
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So if you're in different part of London, you may use the street
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name to work out how to get from you current street to Oxford
St.
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and when you get to the destination street then you're able to
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look at house numbers to determine which house number to go to
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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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route, they will route traffic to a destination Network and when
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traffic gets to that destination Network then house number are
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used or IP Addresses are used to determine which host
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to forward the traffic to
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Now in a similar way, the Network address in this case 10.1.1.0
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identifies a specific network somewhere in the World
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Host Addresses such as 1 in this example identify individual
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devices on that network, so 1 identifies this PC
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on Network 10.1.1.0, each device
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on this network has a unique number, so in this example we have
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5 unique devices in the same way that in our previous analogy
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we had 5 houses on Oxford St. now in the same way
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you wouldn't configure multiple PCs or multiple devices with
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the same IP Address as that would cause confusion and conflicts
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So as an example, if another device wants to communicate with
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10.1.1.1 which PC would it send the traffic to?
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If PC 5 wants to communicate with this host 10.1.1.1
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there are 2 hosts with the same IP Address which device should
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PC 5 communicates with? So devices on a Network or subnet
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which we'll talk more about in a moment have a unique IP Address
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in the same way that a house has a unique house number
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on a specific street, so in the same way
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that we use street names when going from A to B
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routers look at Network Addresses to determine how to get
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to a remote destination, other protocols such as
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Address Resolution Protocol or ARP are used to find
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the house number or host number for a host on a specific street
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or network, so in this analogy we're going from Queen Anne St.
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to Oxford St. and we would use street names for our directions
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as per this example, but once we get to this street in this case
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Oxford St. we would look for the specific house by looking at
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house numbers in the physical world or in the Network world
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we would use a protocol such as ARP to find the specific host
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on that Network, routers do not make routing decisions
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based on IP Addresses and make it based on Network Addresses
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A routers routing table is not populated with IP Addresses
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it's populated with Network Addresses, the destination
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IP Address in a packet is then matched to the routing table
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populated with Network Addresses and then a determination
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is made to decide out of which port or interface on
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the router the packet should be forwarded
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