ARP

Hey, So, what is ARP? ARP stands for Address Resolution Protocol and is designed to discover MAC addresses and then map them to an IP address. So let'see how t In our example, we have four computers and a switch. Our computers have been assigned IP addresses, 192. 168. 0. 1, . 2, . 3 and . 4. Let'say HostA wants to communicate with HostD. We already know the IP address, maybe we typed it in manually. But remember, switches are layer 2 devices, w So for HostA to communicate with HostD, it somehow needs to find out its MAC address. The way it does t HostA will shout out to the entire network saying, who is 192. 168. 0. 4? Tell me your MAC address. And that message is sent as a broadcast. We've already looked at broadcast messages and remember, they will be sent with a destination address of all Fs. And the switch will send t HostB and HostC will look at t 168. 0. 4? That's not me. And they discard the frame. But when HostD sees the message, he says, oh, 192. 168. 0. 4? That's me. Here is my MAC address. And then HostD replies with When our computers learn MAC addresses, they are stored in a table for future use. It lists the known IP addresses and their associated MAC addresses. T From t And now they can both send frames happily. So that was pretty simple. Let'see t I'm going to click the Start button and type PowerShell. Now we're going to need to run t So right click, Run as administrator, accept the UAC. And when that loads, I'm going to start by typing ARP-A. T We have the internet address, w The physical address, w And we have the type. T You can see we have three static entries, and the top one is somet You should be able to recognize t It's the last IP in the network. You should also be able to recognize it by the associated MAC address, w The bottom two are multicast addresses. You can see that from the first six characters of the MAC address. So because my computer already knows the MAC address of 192. 1680. 4, I'm going to run ARP-D192. 168. 0. 4, and t If I then type ARP-Again, we see that entry has been removed. Now I'm going to open up a program called Wireshark, we just need to select the interface and W 168. 0. 4, and t Once the ping has finished, we'll head back over to Wireshark and There's not a huge amount to go through here, but if you're trying this at home, you will T We can filter the capture by clicking at the top and typing ARP. Now we can see our ARP conversation happening. We can see the source and destination. Keep in mind though, Wireshark will try to resolve these MAC addresses to somet If we look at the info col 168. 0. 4. With the first request selected, we can look at the bottom of the screen and see our source and destination MAC address. So host A is the source and the destination is a broadcast MAC address. If we select the second one down, which is the reply, you can see in the info col T If we look at the bottom, we can see host D's MAC address is now the source, and host A's MAC address is now the destination. And now if we go back to our PowerShell window, run ARP-Again, we can see that the entry is now back in the table. Let's look at one more scenario. Here we have two hosts, a router and a web server. If host A wants to communicate with the web server, what will it do first? Well, if we type in the IP address of the web server, host A will compare that IP address with its own. It will look at 192. 168. 0. 1 and 10. 10. 10. 10. And it will realise the web server is not in the same network. ARP requests only happen in layer 2 networks. So we can't send an ARP request to the web server. So what do we do? Knowing it needs to get off the network, host A will do the same ARP request, but not to the web server for its default gateway, w The default gateway is If a device needs to communicate out of the network, it'sent to the default gateway. So we send an ARP request to 192. 168. 0. 254 and request the MAC address. Host B will drop the router will respond with its MAC address just And now we have the MAC address for the default gateway, host A can start to send data. The data will contain the MAC address of the router, but it will have the IP address of the web server. The router will handle t We will go over that's it for ARP. If you liked this video, please give it a like, comment and The support from Other than that,