Subtitles of the Movie

Now let's talk about Subnets. Now if there's one topic that makes network administrators absolutely pull their hair out it's subnetting and subnetting is really nothing more than subdividing a network into multiple smaller networks, okay and let's just take a look at it here. Now this can be complicated. It can be confusing and when I teach it in live classes it can be quite entertaining, but I just want to take you through the basics here in this video and we may do a second part on this, we'll see how it goes, but I want to just go step by step through this and show you the basics and then you can watch these a couple hundred times and then just get the ideas down and you will learn all kinds of tricks about calculating these things, OK? Let's start off with the address that we had: 192.168.2.1. Now I know something about this because all TCP IP addresses, within the address, is a network ID and a host ID. Now think about your address at home, OK, let's say that it's 1501 Main Street, OK, Main Street is part of the address that you share with everybody on your street, this is the same as the Network ID. The 1501 part of your address uniquely identifies your house on that street and that's the way that TCP IP is. We have a network ID and then the host ID uniquely identifies that machine on that network, or on that street if you will. Now, the way we can tell what the street name is, or the network is, in TCP IP is using the Subnet Mask. Notice this Subnet Mask is 255.255.255.0. Everywhere we see a 255 it's saying that this entire octet and just use them positionally, this entire octet is being used for network ID so this is showing us that the 192.168.2 is the network ID and 1 is the host ID. Now you can see this more clearly when we look at the Subnet Mask in binary form because you can see that you have all 1's in each of those three octets. Now keep in mind, if we turn all these 1's on and add them up it comes to 255. Now, we can represent 256 numbers in these 8 bits but keep in mind we start at zero, so while we can show 256 numbers, our first one's zero, so the highest maximum number we can show is 255. Now, that's our Subnet Mask and that's what indicates to us what part of the IP address is network ID and what's host ID. Now the way you will see this written and especially on the exam, is the way you see it here - you will see the IP address 192.168.2.1 /24. This is telling you that if you start at the left side and start counting positions, 24 of the 32 bytes or bits, are being used as Network ID OK and so we convert that back to decimal this is what we get and it's very easy to see how much of this is network ID and how much is host. So what we end up here is one network and then 254 host addresses. Now how did we get to 254? Notice this magic formula right here. This thing will help you tremendously in figuring out subnetting and just all the TCP IP world. Notice that if we take 2 and raise it to the power of the number of positions we have, so 2 to the 8th power, that means 2 times 2 times 2 times 2 times 2 times 2 times 2 times 2. Two times two eight times. Then whatever that comes to and it would be 256, we subtract 2 from that and it leaves us 254. Now why do we subtract 2? Because you can't use the very top address or the very bottom address. The very bottom address is zeros and that's the network ID, the very top address is all ones, which is 255, which is a broadcast address, OK? So, without going any deeper than that that's the way we can figure out what's going on. But let's say that we have this address and this is our network address that we got from our ISP and it gives us one huge network with 254 addresses. We don't have this many computers and we would like to have a number of smaller networks inside our company, so here's what we can do. We can Subnet this. Now let's take a look at subnetting this. So we start back off and 192.168.2.1 and now notice our Subnet Mask has changed. Instead of being 255.255.255.0 notice we see a 224 here. Well let's look at that in binary and check to see what's happened here. Now you will notice that, right over here on the right side we have borrowed three of our host addresses and we're going to use those as network addresses, so we took the network that they gave us, which was these three and we're going to use three of our bits for host addresses as networks, so now how many will that give us? Well, let's back up a minute and let's talk about our notation. 192.168.2.1/27 now. So, if we start from the left over here and count to the right there's 24, 25, 26, 27. So, that's telling us that our Subnet Mask is now 27 of the bits. Well notice now we have 6 networks. Well, how did we arrive at 6 networks? Notice, if I take 2, raise it to the third power; we've got three bits here that we borrowed, so 2 times 2 is 4, times 2 is 8, minus 2 we've got 6 subnets and so we now have 6 networks to work with and what I'm going to do is stop the video here and I will join you in Part 2 of Determining Subnets in the next video.