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Dumb VLAN Question

By clairefromperth ·
Tags: Networking
I’m trying to get my head around subnetting as I venture in VLAN setups home/small business network. I have an all-Ubiquiti network with a UDM-Pro gateway, Unifi 24-port switch, and UAC WiFi device – all rock solid so far. Here’s my really dumb question...

I’m trying to sort out two different approaches.

Approach A: Let’s say I start with a private Class C address like 192.168.143.0 with a 255.255.255.0 (/24) mask – that yields 254 usable addresses. Then I see folks assigning 192.168.10.0/24, 192.168.10.0/24, 192.168.10.0/24,... etc. addresses to their VLANs (VLAN10, VLAN20, VLAN30,... etc.). The subnets have a different address in the third octet.

Approach B: What’s wrong with defining 192.168.143.0/28 (255.255.255.240)? This yields 16 subnets with 14 addressable hosts per subnet with the first three octets the same. Why not assign VLAN10 to a range like 192.168.143.49 - 192.168.143.62; VLAN20 to 192.168.143.65 - 192.168.143.78, etc.

I can understand why Approach A is a neat and organized framework, but you still only have 254 usable addresses, right? If is okay to go this way if you’re sure you won’t need more that 254 hosts? In Approach B, I can see inefficiency in losing addresses at the beginning and end of each subnet, but why is this an option folks don’t seem to use? Am I missing something fundamental about Layer 2 and 3 definitions?

Thanks for helping to straighten me out.
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What are you trying to achieve by segmenting the network?

by spetryschuk In reply to Dumb VLAN Question

I'd take a step back and try to understand what are you trying to achieve by segmenting your network.

Typically, network admins will create different VLANs to segment the physical network into logical groupings. So what are you trying to achieve by having VLAN10, VLAN20, etc? Most often, the VLANs serve a purpose such as separating wired vs. wireless networks, data vs. voice networks, servers vs. workstations. How many VLANs need to be created will depend on how you want to segment your physical network. Remember that each VLAN is a "virtual LAN", or intended to be a separate L2 network.

Next, you'll need to define which Layer 3 subnets are carried on each VLAN. The required size of the subnet will depend on how many devices you intend to connect to that network. Most SOHO and SMB environments use a /24 subnet mask as its easy, even if they don't need all 254 available IPs. Unless you have a specific reason not to use a /24 subnet, I'd stick with that.

As for putting 192.168.10.0/24 on VLAN10, 192.168.20.0/24 on VLAN20, etc, that's just vanity. Easy to remember / document. As long as you've configured the routing correctly, there will be no difference in the performance of the network if you were to use a 192.168.10.0/24 subnet on, for example, VLAN90.

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