The ARP Protocol, or ARP as you will hear it referred to, is one of the most basic Protocols in TCP/IP, and it's one that not many people know about outside the hardcore TCP/IP nerd camp. Now ARP is the Address Resolution Protocol, and what this thing does is pretty fascinating. ARP's job is to find out what a particular computer's Data Link layer address is, or the MAC address on a network card. Now if you remember when we went through the OSI levels, the Network levels, the Data Link layer was the second layer. First was Physical, then Data Link, and Data Link is where the MAC address for the network card and all that is managed. Well as it turns out, a lot of times, on a TCP/IP or just an IP network, same thing, all we know is the Network layer address, the TCP/IP address, the 192 168.3.14, or whatever. And that address is great for TCP/IP, but we've got a problem. We need a little more than that to actually deliver to that address, and why? What's the big deal here? Well, IP in the TCP/IP Protocol Suite, is based on IP addresses. Our underlying network that moves TCP/IP around is the Ethernet network, and that communication is based on the MAC address, and so somehow we've got to get these two together. And that is exactly what ARP does for us. We have to, at some point, allow the Ethernet functionality to kind of coincide with the IP functionality and have both of those, OK? And so ARP's job is to reconcile those. If I only have an IP address, ARP's going to go figure out who's MAC address that is. Now again, ARP operates at that Data Link layer and it only operates on the local network segment that that particular computer or functionality is on. It identifies the MAC addresses associated with a particular IP address and then, here's one of the most important aspects, it stores the information in the ARP cache. And here's why. When ARP needs to do its job it has to a broadcast. It has to more like, broadcast on a network segment, hey, here's the IP address I have. I need to know whose IP address this is and I also need to know what your MAC address is. And when it gets this information it stores it in a cache, which is actually stored in memory on the host machine. They just grab a little slice of the operating system memory and the MAC IP addresses are stored right there in what's called the ARP Cache. Now obviously what this prevents is the next time somebody tries to send something to that IP address it doesn't have to go broadcast, and so forth, it just checks the ARP Cache to see where that particular MAC address is, or see who that it is. Now I'm going to show you the ARP Cache in just a second here and show you how to view it. There are two types of ARP entries that you're going to see in the ARP Cache. The first ones are dynamic, and this is where most of them that you're going to deal with will come from, and this is where TCP/IP Suite sees some an IP address that needs some data delivered. It doesn't have a MAC address and so it calls on ARP to automatically go get that MAC address for us. When ARP gets it, it stores it in the ARP Cache, and it's dynamic. Now I don't want to go to deeply here, but once it resolves it, it puts it in the ARP Cache and it stays there for a certain amount of time, and every time that it gets hit again it stays a little longer, and so forth. So there's - different operating systems do different things here with it. Now a static ARP entry is where we actually go in and manually enter a MAC address mapping to an IP address, and this one is permanent until deleted. Meaning it is stored, don't worry about how, but it will survive restarts on the machine and so forth. So it's going to be there, OK? Now I want to show you ARP. Go to Start Menu, and I'll just type CMD, and press Enter, and what I want you to see here, let me make this a little taller for you. First of all, if I just type ARP space forward slash question mark, this will show you the ARP Command and all the possibilities on it, but the thing I want to really point out to you, that you need to watch for on the Exam, is this A Switch, OK, and the G Switch, and notice G says it's the same as A. But if I type ARP space hyphen lower case A or G, and press Enter, there is my ARP Cache on this machine. Now notice that we have a dynamic MAC address right here that belongs to this IP address, and then we have some statics that were put in by the system itself, alright? Now if you'll notice up here on the switches that I can specify a certain Internet address, I can display the ARP entries for a Network Interface by the IF address, and then with D, I can delete a particular host. I can take something out of the records. And then with the S switch, I can actually add some information in there. Some other things I can do, you can go read through this. They won't go that deep on the Exam, you just need to know what ARP is, what functionality it provides, the TCP/IP Suite, and you just need to know about ARP A and ARP G. This is how we look at the ARP Cache, and just understand how it's working and what it does, and you will be good to go.
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