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DEMO: Subnetting Example B-1

Class B subnetting applies the same foundational concepts as Class C, scaled up to handle significantly larger address spaces with up to 65,534 hosts per network and over 16,000 possible subnets.

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About this video

Class B subnetting builds directly on the same binary-based methodology used for Class C, extended to accommodate a much larger address space. Where Class C limits the host count to 254 per subnet, a Class B address supports up to 65,534 hosts per network and over 16,000 possible subnets, requiring comfort with larger decimal values across multiple octets. The core process remains consistent: determine how many bits to borrow to satisfy both the required number of networks and the maximum hosts per network, then use that boundary to calculate network addresses, broadcast addresses, and usable host ranges. Using a sample address of 141.44.0.0 with a requirement of 32 networks and up to 1,500 hosts per subnet, the correct bit-borrowing point is identified and applied to enumerate the first six subnets. Network addresses are derived through binary counting, broadcast addresses are found by setting all host bits to one, and usable ranges are calculated by incrementing the network address and decrementing the broadcast address by one. A shortcut method for listing all network addresses first and working backward to confirm broadcast addresses is also demonstrated, including how to handle borrowing across octets when subtracting one from round numbers. Verifying results against the expected answer set before moving forward reinforces accuracy and builds confidence with the subnetting process at scale.

What you'll learn

What's covered

Subnetting a Class B Address

Aligned to

Cisco CCNA
1.6 Configure and verify IPv4 addressing and subnetting
CompTIA Network+
1.7 Given a scenario, use appropriate IPv4 network addressing
Cisco CCST Networking
2.2 Identify IPv4 addresses and subnet formats

Key terms

Subnet
A logical subdivision of an IP network, used to organize and segment network traffic.
Subnet Mask
A 32-bit number that divides an IP address into network and host portions.
IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Variable Length Subnet Mask
VLSM
A subnetting technique that allows a network administrator to divide an address space into subnets of different sizes by using different prefix lengths, allocating address space more efficiently than fixed-length subnetting. VLSM requires a classless routing protocol such as OSPF, EIGRP, or RIPv2 that carries prefix length information in routing updates.
Network Address
The first address in an IP network range that identifies the network itself and cannot be assigned to a host. It is formed when all host bits are set to zero.
Broadcast Address
The last address in an IP subnet range, formed when all host bits are set to one, used to send a single packet to every device on that network simultaneously.
Usable Host Range
The set of IP addresses within a subnet that can actually be assigned to network devices, spanning from one address above the network address to one address below the broadcast address. For example, in the 192.168.1.0/24 network, the usable host range is 192.168.1.1 through 192.168.1.254.

Transcript

We're going to subnet a Class B. All the concepts are exactly the same, and really it's just like subnetting a Class C except there are larger numbers that we're dealing with, which may seem a little confusing at first but really is very similar. So what we're going to do is go through another exercise. I'd like you to try it on your own, check your answers, and then we'll go through it together.

The problem

As you can see, we're dealing with quite a few bigger numbers here. We've got the 141.44.something.something, so we're dealing with a Class B address here. We have 32 networks. I didn't have enough space to put them all on there, but we've got a total number of 32 networks. We're just going to do the first six, figure out the first six, and then it ranges anywhere from 1500 hosts per network down to two hosts per network. So that's what we're working with.

Other than that, you can see we made some adjustments, so the hosts go all the way up to 65,000, the networks go all the way up to sixteen thousand, so we made a few little adjustments like that.

So at this point in time go ahead and pause and try this out on your own, and then see how close you came, and after that we will go through it together in case you want a little extra help as you're going through this process.

The answers

Here are the answers, so go ahead and pause again and check to see what you came up with and make sure it's close to what we have here. So go ahead and check your answers. All right, hopefully that's close.

Stepping through the process

Now what we're going to do is step through the process together to see what we come up with.

The first thing that I'm going to need to do is determine how many bits I'm going to need to borrow. I need a total number of 32 networks, so I can get exactly that right there, so 32 networks right there. And then I also need up to 1500 hosts per network, so I can cross that off, or really I can just jump straight to this, this is going to be what we need right there. So this is where I'm going to draw the line. I am borrowing eight bits for this.

All right, next what I'm going to do is start figuring out what my different networks are, so here's where that binary counting comes in. We'll just play this out, and like I say, we're only going to go to this sixth network right here.

Finding the network and broadcast addresses

Next what we need to do is figure out what the network is. So if these are all zeros here for the network address, then what we're going to do — and I ended up putting a zero right there, which this is really just the division line there between the two octets. So what our network is, it's going to be 0.0, so 141.44.0.0.

The broadcast address would then be all ones through here, so if we were to calculate that out, this would be .255, and ones here would be the 4 plus the 2 plus the 1, so four, five, six, seven, so .7. So the broadcast address is .7.255. So the usable range is anywhere from .0.1 through .7.254. So we're going to subtract 1 from that last octet there and add one to the last octet there to find out what the usable host range is. So there we have it.

The other method

I could also do my other method where I figure out all the networks first. So this network is .8, and then .16, then .24, then .32, then .40, and then this is actually .0.0. And then I can figure out what the broadcast is from there by subtracting one.

It looks a little strange when we subtract one like this. .16.0, we're subtracting 1 from it, so we have to borrow from this third octet for the fourth octet to subtract one from it, so that brings that down to .15.255. And this would be .23.255, this one would be .31.255, and this one would be .39.255. And then our next one would be .48.0, so this one would be .47.255.

And then we can figure out the usable range in between, which is going to be .8.1 through .15.254. So I won't make you stay for the rest of this, but you can see we would just calculate for the usable host address, and that is fairly straightforward at this point.

As always, I would just check your work again, make sure that you are good to go, and make sure you have this down before you proceed on to the next set of videos.

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