Can anyone tell me what some of the implications of having two different subnets on the same switch would be if VLANs are not being used?

  • In this case, the risk of spoofing isn't an issue I am worried about. – Kyle Brandt Jun 15 '09 at 14:49
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    This is also useful information for admins migrating a network to a new IP range. – Terence Johnson Feb 13 '12 at 2:37
  • One thing to note, that is hit on a bit in some of the answers below, is that unless you use VLAN or static IP addressing on your clients, they will all pull DHCP from sort of the "default" scope. – Adam Nofsinger Jul 20 '17 at 21:45

Things will work pretty much as you'd expect. At the heart of it, they're just sharing a broadcast domain. The computers in the different subnets won't ARP across-subnet so they will still need a router (or embedded layer-3 entity in the switch) in order to "talk" to each other.

Because they share a broadcast domain there's much less (arguably, none) isolation than if you were using VLANs. It would be easy to ARP and MAC spoof hosts in either subnet from either subnet.

If you're just doing this in a lab scenario it's probably fine. If you truly need isolation, though, in production deployment, you should use VLANs or separate physical switches.

  • It is a production environment, but spoofing isn't really an issue in this case. – Kyle Brandt Jun 15 '09 at 14:50
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    You say that until it is. Either upgrade to switches that do VLANs or buy another switch. Really. – Matt Simmons Jun 17 '09 at 3:12
  • Got another Switch :-) – Kyle Brandt Aug 14 '09 at 21:38

If you don't use VLANs a person could easily just add 2 IPs to their interface say and so that he or she could access both networks.

By using VLANs you can tag the switchports so that any computer configured to only receive traffic from the VLAN will not be able to get any traffic (except the one directed to it and having the correct VLAN) regardless of how the local interface is configured (how many IPs there are on the interface).

In essence:

  • if you trust your users there's no reason at all to use VLANs (from a security point of view).
  • if you don't trust your users VLANs will keep certain groups of users seperated from each other

First, I'm not sure why you would do this for users. The one scenario I can think of is that you are out of IPs in your current user subnet and can't easily extend you current subnet. In this case I think it would be fine to add another subnet. The spoofing thing becomes a non-issue when you are using the IPs this way because both subnets are equal, so you have the same spoofing risk whether using a single subnet or multiple. One question I have here is how DHCP would work. If your DHCP scopes aren't contiguous, and the DHCP server serves IPs based on the "helper" address of the router, wouldn't all the requests go to one scope or the other? I suppose this might become a non-issue if your DHCP server is sitting directly in the broadcast domain, but it's still something to explore.

All that said, I actually do this in production for one of my apps. I have an app that has geographically diverse silos, each silo has its own /27. Those IPs are what I consider to be infrastructure IPs. They belong to those servers. Then I route an additional /29 to the same broadcast domain. This subnet belongs to the application. When I next upgrade hardware, I will build out an entirely new silo with a new /27, then change the route for the application /29 onto it. Since this /29 handles communication with network elements, this allows me to not have to reprogram all the NEs if we get new hardware or new software, and using the same broadcast domain allows me to do it without a dedicated NIC.

  • The 'why' is that our crappy old pos ERP system that is being moved can't change IPs without reinstalling every single client (and other AD issues). Thank you for the DHCP idea, I will have to explore that issue. – Kyle Brandt Jun 15 '09 at 15:05
  1. if you have untrusted users - some of them might spoof ip addresses of those from other subnet. if there are some address rules - they might bypass them. some users from subnet1 might spoof address of router in network b - and eavesdrop into [at least part of ] the communication.
  2. you'll have more broadcast 'garbage' [ arp packets ] - but that should not be your concern if you have few dozens of users and 100 or 1000 Mbit/s link.

We implemented this in our school because we were running out of ip addresses and gave a new subnet to the wireless section, works fine on a 3000 users network, for a quick solution is a plus, I agree we have to create vlans in order to preserve security.

DHCP server (Windows) must have two nic cards connected to the same switch (our is virtual so it doesn't matter) in order to give out ips to the wireless network, you will have to use static IPs on the "old network", it will not work serving two dhcp scopes over the same switch.


I've just spent a couple of years trying to resolve a problem with both a poe phone system and a computer network on the same managed switch. Yes it should work without a VLAN but every month or so it doesn't and would reset the switch, causing endless issues with connected equipment. (phone system resets, router resets and random switch resets) This was a nightmare for us as we were looking for a hardware problem as most accept that a switch can handle this. A dumb switch maybe, but a managed switch doesn't. I tried several major manufactures and they would all reset randomly within a month :(


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