IPv4 addresses are classified in multiple ways, including by class (A through E), by role (network, host, or broadcast), and by special designations such as private ranges, the quad-zero address, and the loopback address. Recognizing each type on sight is a foundational networking skill tested on major certification exams.
IPv4 Address Types
We've been talking about network addresses, we've been talking about broadcast addresses, host addresses, we've been talking about classes — a lot of different ways that we can classify these different addresses. I wanted to, number one, be able to show you some ways that we classify these addresses, but also, number two, introduce a couple of other types of addresses that are out there. It's going to be important, when we're identifying these different types of addresses, to also be able to identify these other types of addresses.
First of all, we classify addresses from a classful perspective. We've got A, B, C, D and E, so they fall into one of these categories here. That's one way that we classify these different address types. I'm just going to start a list of the different ways we classify things, so here's the list that we've got going on.
Another way that we've already been talking about is we have networks, we've got hosts, we've got broadcast addresses. So this is another way of looking at different address types. Not only does it fall under A, B, C, D or E, but we also have these network, host and broadcast. So here's our list that we're creating here, and now I've got network, hosts and broadcasts.
There is also private addressing. Private addressing still falls within class A, B and C. You'll still have a broadcast address, you'll still have a network address, you'll still have usable hosts within these ranges, but these are just another way of classifying these different addresses.
You'll need to be familiar with these different ranges, so be familiar with the class A private addressing, be familiar with class B, be familiar with the class C private addressing. So make sure you've got this down as well. There you have it, we added that to our list — now we have private addresses as well.
Then we have what's called a quad 0 address. This is everything. This is the network of networks. This is to specify all of everything when it comes to IPv4.
Remember, we have these network addresses that can't be used within every single one of our networks — within class A, within class B, within class C — that they have a certain address that specifies a network. That's part of the hierarchy of how things go up: it all trickles up to this quad 0, or 0.0.0.0. So that is something to specify everything within this hierarchy and all other addresses underneath it.
Then we have the 127 network. The 127 we know is a class A, so it's the whole range, it's everything from 127.0.0.0 to 127.255.255.255. However, typically the loopback is usually assigned to the 127.0.0.1, but it does take up the whole spectrum.
So what is the loopback, or the 127 network, or the 127.0.0.1? Loopback is essentially its own interface. If you have a network card, then one of the things it does is it assigns itself the 127.0.0.1, or something within this range, for testing purposes. So if you were to get onto your computer and ping 127.0.0.1, it would then ping itself. Now, that packet wouldn't actually go out onto the network and come back into it. What it's doing is just testing that network interface's network stack on there and making sure that it at least has some basic connectivity. If you were to unplug it, then it wouldn't have that basic connectivity and it would fail.
The other area that we see this in is for things like if you're testing your website, or if you're testing something onto the machine, or if it needs to access something on the machine. An example of that is, let's say you have a web server and you want to test the web page on the web server. One way to do that might be that you bring up a web browser, type in 127.0.0.1, hit enter, and then what it will do is it will access its own web server and then pull that up onto the screen, so you can test that out. So 127.0.0.1 is the loop address, essentially saying it's itself. That is the loopback address, and it's just something to be aware of. It's not something you would ever assign out to a network — it wouldn't work that way.
Here are some ways that we can categorize these different things. We've got class A, B, C, D and E. We've got the network, the host and the broadcast. We've got private addresses. We've got quad zero. We've got the loopback.
One thing that you're going to need to be able to do is look at an IP address and determine what class it's in, determine whether it's a network, host or broadcast, determine whether it's a private address, make sure it's not a quad zero address, make sure it's not a loopback address. You're going to need to be able to identify all of these, so I'm bringing this up all at once, and then we'll do some exercises later on to identify what it is that you're looking at. You'll see test questions on some certification tests about this.
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