2 Vlans on 1 Subnet

August 13, 2009 at 05:40:18
Specs: Windows XP
Hi Experts
We are in the process of merging are voice and data networks and unfortunately I am getting conflicting advice from our network support guy and our telecoms maintenance suppliers. Basically we have an IP range from to PC’s reside on 20.44.4.xx range, and we have been instructed that the IP phones will reside on the 20.44.2.xx range. My understanding is that we have to set up 2 Vlans one for voice and one for data, we are using a HP procurve 2650 PWR switch. Now the telecoms maintainer is telling us that we should not have 2 Vlans on one subnet and this will cause us problems with the Vlans not being able to communicate, without complicated routing etc.. The network team are suggesting this is not a problem and that they have to be on the same subnet with different IP ranges. If anyone could let me know if the telecoms maintainer is correct, or if indeed you can have 2 vlans on one subnet without any issues I would be grateful


See More: 2 Vlans on 1 Subnet

Report •

August 13, 2009 at 08:07:48
Well, you can break up a single subnet into multiples within a VLAN and as long as each subnet has it's own unique gateway and your gateways are all configured correctly and your routing is in order it will work. But to my way of thinking, that's a whole lot more work than necessary if you just make a single, unique subnet your VoIP VLAN.

But hey, I'm not the guy doing the work so if your network guy wants to, let him. If it doesn't work right, then it's his fault and problem, not yours.

I figure you're not the network guy so I have to wonder why you even care enough to write in here and ask?

Report •

August 13, 2009 at 08:22:29
"telecoms maintainer is telling us that we should not have 2 Vlans on one subnet"

telecoms is wrong.

It is a misconception of vlans [based on Cisco training examples of large networks] that each vlan has to have its own subnet.

You don't want the phone and data vlans communicating! if you are doing dhcp then only the dhcp servers port needs to be in both vlans.

Report •

August 14, 2009 at 03:38:56
Thanks for your replies. The reason I ask is because at the end of the day if it does not work I will have to sort it and I want to be as informed as possible.
With regards to the 2 vlans on one subnet they will need to communicate that is part of the reason of undertaken this work. We have a Mitel 3300 system so we will be daisy chaining the phones and PC’s with the phones on 20.44.2.xx and the PC’s on 20.44.4.xx. We will need to access the Mitel apps (web browser) via are desktops. The telecoms maintainer is adamant this cannot be done because they are on the same subnet, any pointer or help would be grateful

Report •

Related Solutions

August 14, 2009 at 08:13:57
Those are two different subnets. Does not appear the issue is vlans but of subnetting. And the provider is right given this circumstance.

Think about this a second. In this situation the phone is a hub. It is also an ip device. So ip has to flow thru the phone to get to the pc.

Any more vlans than the default one can't work here unless these phones also support vlan tagging like you get with a managed switch. Having two vlans on the same wire [to pc not trunking] is pointless imo. Point of a vlan is to isolate traffic. One wire to phone and pc defeats this.

Normally Voip phones are on their own wiring diagram not shared with the data network. This can then support a vlan for phone and a vlan for pcs. Additionally this situation supports two subnets. There is no communication between pc and phone.

This is not your case.

There is no reason I can see to have two subnets in this circumstance. The phone folks are correct in that with two subnets without a router [at each phone] the pc app can't talk to the phone.

Report •

August 14, 2009 at 10:30:53
Everything wanderer said is bang on the money.

I briefly experimented with plugging my PC into my VoIP phone just to see how well it worked and what it took to correctly configure it in case I had to for someone within our network.

Our VoIP phones are VLAN capable. So, I tagged the port my phone plugged into with both separate VLAN's (ie: data (VLAN 7) and VoIP (VLAN 72) ) and plugged the phone in to the switch and my PC into the phones data port. The phone was the base VLAN for that port (ie: PVID = 72, allowed VLAN's = 72, 7)

Both worked perfectly but my PC went from a 1000 Mbps connection on my PC to a 100 Mbps as that's all the phone provided.

Report •

November 10, 2009 at 07:52:24
Absolutely correct on this. There are some phones however that have gig stands on them that can support 10/100/1000 throughput. I know Mitel offers these kinds of devices.

Report •

Ask Question