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Now that we talked about the complex issue of subnetting and supernetting, I want to show you couple of shortcuts that I think you'll find that in come handy, as you start to get comfortable with this concept and do a couple of more problems on your own. This is something that will help you solve real world dilemmas with IP addresses and it will also certainly give you a leg up for testing purposes. Let's say for example that you have the class B address of 131.235.0.0 that's been allocated to your company, your network. That has the default subnet mask of 255.255.0.0. What's possible with this class B address? One network, there it is, with 65000 hosts when we mix and match the 16 bits that can be used to define the host IDs. Now let's say your boss or a question on a test says, all right give me 12 subnets from that 1 network identifier, from that 1 network address give me 12 subnets. Here is the shortcut. Remember that when we work with bits we're working with powers of 2 and so you can say how many bits do I need for 12 subnets. Well you don't have to count it out on your hand, you can always use the calculator, you can keep a binary chart handy, however you want to do this. Let's type in the decimal of 12 and then convert that to binary 1100. So in other words I need 4 bits to represent a 12 in decimal. So that tells me I need to borrow 4 bits from the host portion. So I need to slide my default subnet mask 255.255 4 bits to the right, 1, 2, 3, 4 and now these bits are left over for my host portion. So my new subnet mask, my custom subnet mask looks like this 255.255.240. When we convert this octet to decimal, decimal equivalent 240 or we can represent this in CIDR notation with the forward slash 20. This would let us know that we're using 20 bits set to on in the subnet mask. How many hosts will be on each of those new subnets? Well let's take a look here, if I have 8+4 bits to mix and match that leaves me with 12 bits to mix and match. So let's go back to the calculator, go back to decimal, clear this out here, so 2 x to the y 12 equals 4096 and remember to subtract 2, 4094 posts for each and every one of these new subnets that is creatable. So that's one way you can use the shortcuts. The reverse of this is say that some comes to you and says I have a need for 1500 clients for each subnet, I've got this default network number that I've been assigned, which can have 65000 hosts but I don't need 65000 hosts on one network, but I do need at least 1500 clients on each of my subnets. So how many subnets are possible here? Well how many bits do you need to represent 1500? Put that back into the calculator and what do you get? Clear this out, 1500 in binary and you need lots of bits, you need 11 bits. So you need to make sure that you leave 11 bits available to mix and match. Which in this case there's 8, 12 and in fact there we go, 8, 9, 10, 11, so my new custom subnet mask would look like this 248 or 21 in CIDR notation. How many hosts per subnet, 2 to the 11th or what is that in decimal, clear, 2 to the 11th, 2048 minus 2, 2046. How many subnets are possible? Since I'm borrowing 5 bits to mix and match out of my one address, my one class B address, I can rearrange these 2 to the 5th number of ways and that is 32. If I need 1500 clients for each subnet and I've got a class B network, I need to borrow 5 bits and that leaves me with 32 subnets and on each of these 32 subnets, I can define 2046 hosts. These are couple of the ways that I like to communicate and if you look it up in different books or go to other sources you might find other tips and tricks, but these are two very effective ways that I found for working with subnets and supernets. And again remember when you're working with supernets, you are just reversing the direction of this subnet mask. You're combining multiple networks into a single entry by sliding the subnet mask the other way. But either which way, these shortcuts still work in that you can work with powers of 2 to quickly define how many hosts you need on the subnet, what the subnet mask needs to be and how many clients can be on each subnet. So hopefully these will come in handy as you learn this, understand this and hopefully it will come in handy as you sit for the 291.