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TechKnowSurge Catalog Videos DEMO: VLSM Box Visualization
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DEMO: VLSM Box Visualization

This topic covers Variable Length Subnet Masking (VLSM) using a visual block method to efficiently allocate subnets of different sizes from a single Class C address space. It demonstrates how to assign the correct subnet mask for five networks with varying host requirements.

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About this video

Variable Length Subnet Masking (VLSM) is a subnetting technique that allows a single IP address block to be partitioned into multiple subnets of different sizes, each tailored to the number of hosts it must support. Rather than applying a uniform subnet mask across an entire network, VLSM allocates only the address space a given segment actually needs, preserving the remaining space for other subnets. A binary block visualization method makes this process concrete by representing the total address space as a subdivided grid, where each split corresponds to borrowing one additional bit from the host portion of the address. Starting with 192.168.20.0/24, the approach addresses five networks in order from largest to smallest host requirement. The 98-host network receives the first half of the address space as 192.168.20.0/25, providing 126 usable addresses. The 45-host network is assigned 192.168.20.128/26, yielding 62 usable addresses. The 25-host network takes 192.168.20.192/27, with 30 usable addresses, and the two remaining two-host networks are allocated 192.168.20.224/30 and 192.168.20.228/30, each providing two usable host addresses. For each subnet, the network address, broadcast address, and full usable host range are clearly defined by the boundaries established during each subdivision. This ordered, largest-first allocation strategy is fundamental to effective VLSM design, ensuring that no subnet assignment overlaps with another and that address space is consumed as efficiently as possible across the entire plan.

What you'll learn

What's covered

VLSM Box Visualization

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
CompTIA A+ Core 1
2.6 Compare and contrast common network configuration concepts

Key terms

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.
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.
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 go ahead and do another example, but this time we'll use the box to visualize how to break this down into varying length subnet masks.

We'll need to provide for five different networks here. The first one will have 98 hosts in it, the next 45, the next one 25, and then two networks that just have two hosts each. We have this 192.168.20.0/24 address space to work with.

The First Network: 98 Hosts

We'll go after this first network first, since it's the largest one. What I'm going to do is break this down until I get at least 98 hosts in it. If I were to break this down right here — first of all I've got to go vertical, so I'll split halfway down the center here — I would get everything from 0 through 127. If I had to break that in half again it wouldn't be enough hosts, so this is the one I want. So I'm going to use up this box of addresses first.

I'm going to do 192.168.20, and this is going to be a dot zero slash — and I'm borrowing just one bit here, because I split the table just one time, and so this is a slash 25.

The Second Network: 45 Hosts

Next I need 45 addresses. If I were to use this whole other block over here, not only would I not have enough for the rest of my networks, but I'd have a lot of excess addresses. But if I split this in half again — so I'm going to split this in half again — so at this point in time I'm now down to a slash 26. If I borrow two bits here, then I've got this whole box that I can work with. It's 64 addresses, 62 usable addresses, and that's going to fit my 45. If I were to split it again in half, then I would get 32, and that's not enough.

So my next one here is 192.168.20, and the first address in this range is dot 128, so that is a slash 26.

The Third Network: 25 Hosts

My next one here is a 25. This down here, the bottom right hand corner, that's what I have to work with now. So what I'm going to do is, if I use the whole block up it wouldn't allow me extra for the rest of the networks, so what I'm going to do is break this down again in half, which would give me just the right amount here — that would give me a range of 32.

So I'm going to do 192.168.20 dot, and this is going to start with a 192. So 192, and this will be a slash — what do we have, we have a 25, 26, 27 — so slash 27.

The Last Two Networks: Two Hosts Each

Next we just need two hosts, so rather than — we could split this in half, or actually I need to split it in half vertically first, and split in half again, and then split in half again to find this range right here. So now we're at a slash 30.

The next one here is going to be dot 224, so 192.168.20.224. We said that's a slash 30, and there's the address right there, and so it's just using this block right here. The next one I can see here is going to be 192.168.20.228, and that will be a slash 30 as well.

The Ranges

So now I've got that range. For each one of these I've got the network address, I've got the broadcast address, and then all of the usable addresses in between that, and I can do that for every single network.

  • This is going to be 128 to 191, and the usable host addresses are going to be everything in between there.
  • Same thing, 192 to 223, usable hosts everything in between there.
  • For these, 224 to 227, usable hosts are going to be these two addresses.
  • For this network, 228 to 231, and there's the usable addresses for that range.

So there you have it, we have figured out how to break this apart using this box visualization method.

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