Subtitles of the Movie

Here's a topic that will be tough to cover in a single module but we'll do what we can here. You're going to probably need to do this over and over again, maybe 4, 5, 6, 7, times to fully understand or even to kind of understand both the topics of subnetting and supernetting. It's something that if I'm teaching a class I'll do 4, 5, 6 examples and then finally start to see the light bulbs turn on in people's heads. Nevertheless, talk about it here, you can always replay this module to your hearts content, both of these have to do with defining custom subnet masks, so that you can either divide up networks or combine networks. When you subnet, you borrow bits from the host portion to make a new custom subnet mask. In other words you slide the subnet mask bits to the right. You add ones to the subnet mask on the right hand side. It's done to divide a single network into many and thereby if you have multiple networks divided by routers, you reduce the impact of broadcast traffic because again routers don't pass broadcast traffic. Supernetting is just the opposite of this. This is sliding the subnet mask to the left to take away ones from the subnet mask and in doing so you combine many networks into one. And the best thing to do here to get used to subnetting and supernetting is just to start taking examples and the example that I use is the one of subnetting. Let's say for example that you are given the network ID of 192.168.2.0 that is a class C network, the default subnet mask is 255.255.255.0 and that gives you how many hosts per network that allows you 254 hosts for that network. What if you wanted to divide that single network up into multiple networks? What can you do? And by the way just to set the table here, I'm using a very simplified example and you probably wouldn't divide a class C network up like this unless it was for very specific purposes but it's just easier because I can write out all of the binary equivalents here as you will soon see. So just wanted to throw that out before we get going. So on our given network we can define 254 hosts. Now to make more subnets from this single network ID, I borrow 5 bits from the host portion. So I'm going to slide this subnet mask which visualize all ones followed by all zeros, I'm going to slide it down 5 bits to the right, so my new subnet mask looks like this, ones, ones, ones and now 5, I'm sliding the subnet mask to 5 and then these are going to mask out my network numbers, and these are going to be left over for the host numbers. So if I were to convert this into decimal 255.255.255.248 I mentioned in previous modules that as long as this follows the rules all ones followed by all zeros, you're in great shape as far as the subnet mask. Will you ever see this in a subnet mask? 131 for example? No you won't because 131 if we were to convert to binary it would not be all ones followed by all zeros. So in the subnet mask you'll see numbers like 240 or in this case 248.