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As we start to look further at IP addresses, it becomes necessary to identify the IP address classes. And we'll be dealing in this module with the class A, class B, class C networks. These categories define a range of the networks that can be used for a given class and also the number of the hosts per network in each of these classes. They are defined also by the default subnet mask they use and to the computer they're defined by the bits in the first octet as we'll soon see. We're going to be able to look at an address that's starts with like 131, for example and say oh that's a class B address. But really we need to understand things the way that the computer does. First of all a class A network. In a class A network the first bit is always, always, always set to 0. The default subnet mask it uses is 255.0.0.0. With the first bit set to zero, the lowest number is off, off, off, off, off, on, which would give us 1, the highest is here and that gives us 64+32+16+8+4+2+1=127. However there's a caveat which we'll talk about in the next module, can't use the 127, so we use 126. That gives us the range, 1 through 126, 126 networks. How many hosts can we define on each of these networks? How many hosts can we put on the 9 network? We can put 16 million. How do I know that? Because I have 24 of the 32 bits, I have 24 that I can mix and match to define my host IDs. And if I go to my calculator I can quickly do that shortcut by doing 2 x to the y, 24=16 million, and then actually I have to subtract 2 from that number and I'll discuss that as well in the next module. So approximately 16 million hosts per network. The class B network. The first bits are always set to 1 and 0. The subnet mask used in the class B network is 255.255.0.0. so we can use the first two octets to define the network IDs. Its range starts with 128 and goes to 191.255. To show you what I mean here, let's go back to our binary equivalent, now these two things are locked in stone, 1, 0 and they are set. So the lowest number 10000 would be 128, and the highest number in that first octet 10, so 128+32+16+8+4+2+1 is 191 and remember we can use that second octet as well. So an example of a class B network would be the 128.200.0.0 network. We can create in the class B range 16 thousand different networks by mixing and matching all the other bits we have left. In other words with these two locks in place we have 6+8 we can use, so 2 to the 14th. We have 16 bits to mix and match to define the hosts per network, that leaves us with about 64 thousand and again you can go back to the calculator 2 to the 16th, remember to subtract 2, it's 65534, but that specific number isn't that important. In a class C network the first bits in the first octet are always set to 110. The default subnet master class C network is 255.255.255.0. So that only leaves us with the last octet where we can mix and match ones and zeros to define the hosts on each of these class C networks. But we have a wide range of class C networks available and the range is in this ballpark right here. Starts with 192.0.0 network and goes to the 223.255.255 network. How do we know that? Let's apply our rule here, this is going to be set to one, that's going to be set to zero, these things are locked in place, and now the smallest number we can have 110 all the rest zeros, which would give us 192 and the biggest number we can have turning all these on 223. How many networks are there in the class C range? There are 2 million or approximately 2 million. We lock out the first three and then we have 5+8+8 bits to mix and match, how many hosts per network, 254. We have 8 we can mix and match to define all of the hosts on any one particular class C network, and remembering our rule here, let's clear this 2 x to the y, 8=256, remembering to subtract 2, 254 hosts on every class C network. There are lots of charts you can find and lots of books that help you keep this handy, this information handy, you can probably do a search on the Internet and pull down some kind of a chart or table that would help you have this handy. I wouldn't say it's crucial to memorize the different ranges but it certainly crucial to become familiar with these class addresses and what all is in these classes. Class A network defined by this, the default subnet mask and so on class B network defined by this. Here's the range because you really need to become comfortable enough with IP addressing, to see a number like 220.100.5 and understand that that is a class C network that you are dealing with and by default you can only put 254 hosts on that network. And that becomes crucial when you are planning a real world networking, when you are answering questions, on maybe the 291 exam, let's say your boss wants you to put X number of hosts on this network or using this IP address, how can you make that happen.