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Looking at a fiber ring network of 11 switches on the same subnet, with 3 types of devices per module (device 1, 2 & 3).

control room switch         192.168.11.100
module 1 ethernet switch    192.168.11.101
module 2 ethernet switch    192.168.11.102
module 3 ethernet switch    192.168.11.103
module 4 ethernet switch    192.168.11.104
module 5 ethernet switch    192.168.11.105
module 6 ethernet switch    192.168.11.106
module 7 ethernet switch    192.168.11.107
module 8 ethernet switch    192.168.11.108
module 9 ethernet switch    192.168.11.109
module 10 ethernet switch   192.168.11.110

Isolating broadcasting messages/ communication between device type using VLANs:

VLAN 1 - device 1
VLAN 2 - device 2
VLAN 3 - device 3

Devices are not required to communicate across the VLAN (eg Device 1 does not need to talk to Device 2), but the controller needs to be able to access all of 3, which is why one subnet is preferred as opposed to 1 to 1 VLAN/Subnet relationship.

I've done a lot of searching on the internet and there is a lot of debate on whether it is possible to have multiple VLANs on a single subnet. Or if it is better practice to have one subnet per VLAN?

Can anyone clarify...

Is it possible to have multiple VLANs under one subnet?

On the assumption it is not possible..

Other considerations:

  1. VLANs and multiple subnets (not preferable)
  2. Access lists - Port Access Control table with Moxa switches - assigning port to MAC address?
  3. Using multicast filtering: IGMP snooping -register hosts with multicast groups, IP querier L2 switch with lowest IP address, forwards packets to registered ports only.

Is it possible to use IGMP snooping on one LAN without the use of separate VLANs?

Correct me if I'm wrong but if the control room switch is the IGMP querier, it will send queries to all module switch ports, and reports will be sent back by the ports wanting to register to the multicast group. When data belonging to a multicast group is received by the control room switch, it will send information to all the ports that have registered in that multicast group.

Does this also work the other way round - sending data from the module ethernet switch to the control room switch? For example data from device 1 on module 10, only needs to be received by the control room switch. Can IGMP snooping be used here to send packets to the control room switch port designated to that multicast group? How will the packet be forwarded if the module 10 switch is the source but not the querier?

Can IGMP snooping be used to trasmit packets to registered ports from any switch or just from the querier switch?

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  • This sounds like you are looking at it the wrong way. Look at Private VLANs. With a Private VLAN, all the hosts are on the same VLAN, but they cannot communicate with each other.
    – Ron Maupin ♦
    Jan 12, 2017 at 15:50
  • What kind of switches do you have? That will affect your options.
    – Ron Trunk
    Jan 12, 2017 at 15:55
  • @RonMaupin I have a look into private VLANs thanks!
    – L Marfell
    Jan 12, 2017 at 15:59
  • @RonTrunk moxa managed switches: EDS series
    – L Marfell
    Jan 12, 2017 at 16:00
  • Unfortunately, Moxa switches don't have that feature. You can write access lists (ACL) to restrict traffic between devices to do the same funcionality as private vlans. It's a bit of a kludge, but I think it's your only option.
    – Ron Trunk
    Jan 12, 2017 at 16:25

1 Answer 1

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Switches have no concept of subnets. The term is virtual LAN's, VLANs. The management interface on each switch certainly can be on the same VLAN (and subnet). The ring topology requires some flavor of STP to handle the loop prevention of frames.

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