From the course: CompTIA Network+ (N10-009) Cert Prep

Special IP addressing

From the course: CompTIA Network+ (N10-009) Cert Prep

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Special IP addressing

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- There are a number of very special IP addresses that you need to be able to recognize for the exam. So, I want to go ahead and in this episode, run through all of these. So, the first thing I want to talk about is possibly the most complex one, and that's what we call private IP addresses. There are three different groups of IP addresses that are only used in private networks. You'll never Google to one of these addresses. You're never going to get on your Office 365 to these addresses. These are used on internal networks for systems that never do any sharing. Well, be careful with that. They might be sharing within their local network, but they're certainly not sharing out to the internet. There are three different groups of private IP addresses. The first group is any IP address that starts with a 10. If you see an IP address that starts with a 10, it is a private IP address. You'll never see this out in the real wild internet. The second group is any IP address that goes from 172.16.whatever to 172.31.whatever. This group of IP addresses is also only for private networks. And the last one, I'll bet some of you out there will recognize. Any IP address that starts with 192 168, and then the other two values don't really matter, but when you see that 192 168, what you're looking at in that situation is a private IP address. Now a lot of people go, "Now, wait a minute, Mike, "I have seen that 192 168, "and the computer I was on was able to get to Google." That's right. But nobody could get to you. Understand there's a big difference there. When you're using these private IP addresses on computers that get out to the internet, You're usually, well you're not usually, you are working behind a very specific type of device covered in other episodes called a NAT device, which kind of just hides you from the big scary internet. Okay, the next type of special address I want to show you is called a loopback. Now, a loopback means, basically, talk to myself. And back in the old days, we would use, I've got a device right here, can we get a little peek at that? So this is what's known as a loopback, and this little loopback adapter is something I'll show in other episodes. But what we would do is we would plug that in, and then we do something like this. Watch this. So, I'm going to go ahead and ping... (keyboard clicking) See this right here? This is the loopback address, 127.0.0.1 in IPv4 that allows us to reference ourselves. So, basically, what you're doing here is you're just pinging yourself. Loopback in the old days when you had little adapters like that, it would actually go out and come right back in. And it was really a good way to be able to test network cards. That's really no longer true. Loopback doesn't have nearly the usage that it used to have, because most network cards today if they see a loopback address like that, they just test their own internal circuitry. Now, 127.0.0.1 is the loopback, but watch this. So, I'm going to type in 127.0.3.67, and that works also. So, what really happens... Now for the exam, make sure you know it's 127.0.0.1, but in reality, you can type in 127.anything and it will always loopback. So, in my opinion, 127, the entire network ID of 127 is the loopback. But you know, for the exam, 127.0.0.1. Now we also cover IPv6, and we do tons of this in other episodes, but right now what I want to show you is the loopback address just for IPv6. So I'm going to backspace this out. ::1. Now watch this. Now you'll see how it replies back. And that is, if you have IPv6, you can use that as your loopback address. Okay, the last one I want to talk about is something called APIPA. Now, most of these computers we use today use DHCP. We don't manually type in IP addresses into our individual computers. So when this guy boots up in the morning, the first thing he's going to do is go, "Hey, somebody gimme an IP address," and a DHCP server somewhere out in the world is going to respond back with IP address information for him. However, there are situations, it's rare, where a DHCP server goes down. The idea of an APIPA address simply means look, if you can't get a DHCP server, we will give you an address. So at least you can talk locally, you won't be able to get on the internet, but you'd be able to share folders and files and stuff like that. APIPA addresses always start with 169.254. And then each individual computer will randomly generate the next two values. So, there is a 65,536-in-one chance that somebody could roll the exact same two values, but it doesn't happen very often, and that's why Mike doesn't play the lottery. What I've done here, is I've opened up good old Control Panel, and I'm under my Network and Sharing Center. So, right here where it says Change adapter settings, so this is the card I'm interested right now, the built in ethernet for this guy. Oops. And let's go to Properties. And down to TCP/IP version 4, and Properties again. Now, this is where you set all your DHCP information, but there's another tab there that nobody ever looks at. See this one right here? Alternate Configuration. See what it says? Automatic private IP address. And that's where that little setting comes from. If you leave that as it is, it's going to give you the APIPA, Automatic Private IP Address. It will give you an APIPA, address if the DHCP server's down. And you're going to see scenarios on the exam where, you know, Janet is working on this stuff and she can't get to anybody but local stuff, she runs a command and sees that her address is 169.254.something. It means the DHCP server is down. For the exam, make sure that you're comfortable with these different types of private IP addresses, with loopback addresses, and with APIPA, because not only are they on the exam, but this is an important thing for any good network technician to know.

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