A visual box-splitting method offers an alternative approach to subnetting, allowing network engineers to calculate subnet ranges, custom masks, and usable hosts by repeatedly halving an address space.
Visual Subnetting Method
It's always nice to have options on how you break things down, so I'm going to show you one other way that perhaps you like more than the table. I'll show it to you and then you have an option of which one you want to use.
What we have here is a box. In this box, imagine there's a zero and a 255, so that's your full address range of one of your octets. If I want to borrow something, it's like splitting this in half. I'm turning it into two different networks, and the first network is from 0 to 127 and the second network is from 128 to 255. So I have just borrowed one bit there.
Now imagine we want to do that again. I'm going to split this in half once again, and now we've got from 0 to 63, and we've got 64 through 127, and we've got 128 through 191, and 192 through 255.
In order for this to work out right, I always have to do vertical first and then horizontal. So I'm going to split this vertical, and now that I've split it that way, it's going to be 0 through 31 and 32 through 63, and this will be 64 through 95 and 96 through 127.
I'll just start using my slides now as we split this apart, because it's going to get more and more. Now we have eight networks here, because we borrowed three bits. Then we divide that in half again, and now we have 16 networks and there are 16 hosts per each network. And we can break that in half again, once again going vertical first, and you can see that we keep doing this. What we end up with is this square right here.
This square is really what we want to get to. What you can do now is subnetting with the square. Depending on what you need — let's say you need seven different networks — you can say, all right, there's two networks, you just created two networks, and then four networks, then eight networks. So now you've just created eight networks here.
You know the first one is from 0 to 31, and all the usable hosts are right in between, which would be 1 through 30. The next network would be 32 to 63, and all the usable hosts would be right in between. The next network would be 64 through 95, and all the usable hosts are everything in between.
If you needed more networks, you would just split this again. And if you need more networks, you would split it again. And if you need still yet more networks, you would split it again. So you could do this down to however many networks you're going to need to work with.
Here's an example right here. We have 192.168.50.0. The address class of that is a Class C. The default subnet mask is 255.255.255.0.
The number of borrowed bits is something we need to figure out. We need seven subnets, so there's two, four, eight — that would do it right there. So we're going to borrow: we divided it once, we divided it twice, we divided it three times, so we're going to borrow three bits. The custom subnet mask for that would be 255.255.255.224.
For the total number of subnets, we can count it: one, two, three, four, five, six, seven and eight, so there are eight total. Hosts per network is 32, and usable hosts is 30.
So there you have it, there's our answer. And if we need to write out what the ranges are, then we have that information as well.
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