Original subtitles

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On the DHCP server, we could use commands such as, show ip dhcp binding 2

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to see the client ID that's associated with an IP address. 3

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So this IP address is being allocated 4

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to this client ID here and it was allocated automatically. 5

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Now that client ID isn’t very easy to read. 6

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So what I’ll do is go into the interface on the client 7

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and shut that down, in this example a DHCP release message was sent 8

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to the DHCP server before the interface was shut down. 9

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So when we look at the binding on the DHCP server, no binding now exist. 10

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So ip dhcp client and we'll specify a client-id in ascii 11

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and you'll simply set it to PC 2 and hit enter 12

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and then use the command no shut to bring the interface up. 13

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So we see a discover message, there’s an offer request from the client 14

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and an acknowledgement from the server. 15

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show dhcp lease what we can see here is the client ID is set to PC 2 16

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and this is the client ID hexadecimal dump. 17

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On the server show ip dhcp binding notice the user name is the same as on the client. 18

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This can be converted from hexadecimal to ASCII and you’ll get PC 2. 19

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As an example, the hexadecimal value 50 is the decimal value 80 20

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and equates to capital P 21

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in the same way hexadecimal 43 is capital C and hexadecimal 32 is value 2. 22

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So this equates to PC 2 which can also be seen on the server. 23

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So I wanted to shut that interface down, and show you Wireshark. 24

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So I'll shut it down, I’ll start a Wireshark capture. 25

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On the server router 2 and on router 1 26

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I'll no shut the interface and let’s see if we get some DHCP messages. 27

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So firstly, here's a DHCP discover message. 28

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Bootstrap protocol discover, so DHCP discover 29

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source port is UDP port 68, destination port is UDP port 67 30

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it’s a boot request or DHCP request 31

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notice the client MAC address is A,B,C as configured, the client IP address is 0. 32

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The PC doesn’t know what IP address to use 33

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and it doesn’t know about any DHCP servers. so it sending a message 33

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to the broadcast address 255.255.255.255 so you can see it’s a discover. 34

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There's other information here including the host name 35

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of the device making the request, as well as a client identifier. 36

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Scrolling down a bit further, we can see the offer from the DHCP server. 37

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So this is from the server to a broadcast address 38

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because the client doesn’t have an IP address, it needs to be a broadcast. 39

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So it's a DHCP offer, offering this IP address to a client with this MAC address. 40

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We can see information such as, once again it’s an offer 41

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client identifier, so that's the server IP address 42

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lease time is 7 days because we configure that manually. 43

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We can see the subnet mask being /24, we can see option 3 44

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default gateway, we can see option 6, DNS server 45

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and we can see option 150 TFTP server. 46

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The client then request that address 47

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the clients still doesn’t have an IP address 48

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so it sends it from a source IP address of 0.0.0.0 to a broadcast 49

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and it's asking the server through the request 50

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if it can have this IP address that was previously offered 10.1.1.14 51

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the server then acknowledges, that the client can have that address 51

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notice it's still a broadcast 52

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so the server is telling the client that, yes they can use this IP address. 53

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so the client, with this Mac address once again 54

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has now being drawn to the right to use that IP address 10.1.1.14 55

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so the client can now start using this address 56

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and we can see that on the client, that it was assigned this IP address.