Original subtitles

84% So an IP Address consists of 2 main portions

84% we have the Network Address Portion also known as the Network ID

84% this identifies a specific network, routers maintain routing

84% tables that contain Network Addresses it's important to realize

84% that routers build their routing tables based on the Network

90% Address and not on the host address, so they do not route packets

84% from 1 interface to another interface based on IP Addresses

84% they do their routing based on Network Addresses, so they will

84% look at the destination IP Address in a packet and match that 10

84% to a Network Address in their routing table to determine how 11

84% traffic is routed. So an IP Address consist of the Network 12

84% portion as well as the host portion which is also called 13

84% the Host ID, this identifies specific end point on a Network 14

84% such as a server, a printer, a PC, an iPhone, an iPad or some 15

84% other type of device including IP Phones, these are essentially 16

84% devices that need to communicate with each other, so PC may be 17

90% communicating with the server, you may be using your iPad to read 18

84% the news, so you'll be surfing to cnn.com or usatoday.com 19

84% or the BBC, so your iPad is sending traffic to that server and 20

84% the server is returning traffic to your iPad, so the iPad and 21

90% the server will have a TCP session established at Layer 4 but for 22

84% the iPad to communicate with the server, routers need to route 23

84% traffic to the server and back again 24

84% So here's an analogy to explain Network and host 25

84% in cities throughout the world you have streets, so as an 26

90% example we have Oxford St., on the street we have multiple houses 27

90% or multiple offices but in my example let's assume we have houses 28

84% on the street, each house has an individual house number 29

84% so we have in this example 1 Oxford St. 2 Oxford St. 30

84% 3 Oxford St., 4 Oxford St., 5 Oxford St. and so forth and so on 31

84% each house number uniquely identifies that specific house on 32

84% this street, now you wouldn't have 2 houses with the same number 33

84% on the same street, that would be very confusing because perhaps 34

84% you want to meet your friends, the Joneses at #1 Oxford St. 35

84% but you may end up spending your day with the totally 36

84% different group of people if you have 2 houses with the same 37

84% number, in this example we have 2 #1 Oxford St. houses 38

84% Which house is the correct house? 39

84% Now that's not desirable, when you go to 1 Oxford St. you want 40

84% to make sure that you are going to the right house to meet 41

84% your friends, rather than going to some other house and hence 42

84% house numbers need to be unique on a street 43

84% in a City, in the World. Now continuing with our analogy 44

84% when working out how to get to remote destination 45

84% in the physical world, let's say you want to go to Oxford St. 46

84% in London, in the UK and you've never been before 47

84% you may use a mapping technologies such as Google Maps to find 48

84% out how to get to that specific street, now when you're doing 49

84% this you don't look at individual house numbers but rather look 50

84% at street names to determine your way to get to that destination 51

84% So if you're in different part of London, you may use the street 52

90% name to work out how to get from you current street to Oxford St. 53

84% and when you get to the destination street then you're able to 54

84% look at house numbers to determine which house number to go to 55

90% technologies such as Google Maps are obviously really intelligent 56

84% and know the house numbers and will guide you to the right 57

84% part of the street, but for this analogy thinks of how routers 58

84% route, they will route traffic to a destination Network and when 59

84% traffic gets to that destination Network then house number are 60

84% used or IP Addresses are used to determine which host 61

84% to forward the traffic to 62

84% Now in a similar way, the Network address in this case 10.1.1.0 63

84% identifies a specific network somewhere in the World 64

84% Host Addresses such as 1 in this example identify individual 65

84% devices on that network, so 1 identifies this PC 66

84% on Network 10.1.1.0, each device 67

84% on this network has a unique number, so in this example we have 68

84% 5 unique devices in the same way that in our previous analogy 69

84% we had 5 houses on Oxford St. now in the same way 70

84% you wouldn't configure multiple PCs or multiple devices with 71

84% the same IP Address as that would cause confusion and conflicts 72

84% So as an example, if another device wants to communicate with 73

84% 10.1.1.1 which PC would it send the traffic to? 74

84% If PC 5 wants to communicate with this host 10.1.1.1 75

84% there are 2 hosts with the same IP Address which device should 76

84% PC 5 communicates with? So devices on a Network or subnet 77

84% which we'll talk more about in a moment have a unique IP Address 78

84% in the same way that a house has a unique house number 79

84% on a specific street, so in the same way 80

84% that we use street names when going from A to B 81

84% routers look at Network Addresses to determine how to get 82

84% to a remote destination, other protocols such as 83

84% Address Resolution Protocol or ARP are used to find 84

84% the house number or host number for a host on a specific street 85

84% or network, so in this analogy we're going from Queen Anne St. 86

84% to Oxford St. and we would use street names for our directions 87

84% as per this example, but once we get to this street in this case 88

84% Oxford St. we would look for the specific house by looking at 89

84% house numbers in the physical world or in the Network world 90

84% we would use a protocol such as ARP to find the specific host 91

84% on that Network, routers do not make routing decisions 92

84% based on IP Addresses and make it based on Network Addresses 93

84% A routers routing table is not populated with IP Addresses 94

84% it's populated with Network Addresses, the destination 95

84% IP Address in a packet is then matched to the routing table 96

84% populated with Network Addresses and then a determination 97

84% is made to decide out of which port or interface on 98

84% the router the packet should be forwarded