Single VLAN can support multiple subnets

[mahesh18]

Hi all,

I was reading cisco book where it says Sinle vlan can support multiple subnets.Because switch ports are configured for vlan  number only and not a network address any station connected to a poprt can present  ant subnet address range.

if someone can please explain me this with example.

thanks

mahesh

[Reza Sharifi]

Hi Mahesh,

Yes, it is possible to have multiple subnets in the same vlan. You would just assign multiple subnets under the same vlan interface but it is not a common practice at all.  The cleanest way is to have one subnet per vlan to keep you broadcast domains separate.

HTH

Reza

[Federico Coto Fajardo]

As Reza said, the recommendation is to have:

1 VLAN = 1 IP subnet = 1 Broadcast Domain

VLAN is a L2 concept, therefore technically is possible to have multiple IP subnets (L3) on the same VLAN.

However this practice is not recommended at all.

Federico.

[dscarfe]

There some legimate reasons to have multiple IP subnets on the same vlan.  For example, you might need to increase the size of a subnet due to address exhaustion.  If an adjacent subnet isn't available to supernet with, one possible solution is to add another subnet as a secondary IP interface.  A large drawback to this solution is that hosts in the same vlan, but in different subnets, would require a router to communicate.

While this might serve well as a migration tool, it isn't the cleanest way to configure a network.  It is still better to have a 1:1 relationship between vlans and ip subnets.

Later,

Dave

[mahesh18]

Hi,

thanks for replying.

if you can explain me with example how can we do this?

thanks

mahesh

[Federico Coto Fajardo · Accepted solution]

As stated above, there are some circumstances where you can have multiple IP subnets in the same VLAN.

I think however, that we come to the same conclusion where you should have a 1-to-1 relation between VLAN and IP subnets as a recommended design.

If you find yourself in a situation where you must aggregate different subnets on the same VLAN, my opinion is to consider a redesign.

VLANs are L2 and are just a concept that switches know about and IP subnets are L3.

So, as I said, technically you can have multiple IP subnets on the same VLAN but again, not recommended solution.

Federico.

[mahesh18]

Hi thanks for great reply

i mean to know command  syntax for multiple subnets on single vlan

this i need to know only?

thanks

mahesh

[Federico Coto Fajardo]

Depends on your setup.

For example, if you have a L2 switch.... computers can connect to ports on the same VLAN, and the computers belonging to different subnets.

No extra configuration needed.

Federico.

[Reza Sharifi · Accepted solution]

Hi Mahesh,

Take a look at the config example I have below:

interface Vlan10
ip address 20.20.20.1 255.255.255.0 secondary
ip address 30.30.30.1 255.255.255.0 secondary
ip address 10.10.10.1 255.255.255.0
end

Again, not recommended but this is just for your info:

HTH

Reza

[mahesh18]

Hi all,

Many thanks for all for answering the question

thanks

mahesh

[rabbdavid]

Hi, I am trying to simulate that on PT but I can't give command ip add second network gateway ip add + subnetmask secondary

Basically I can put only one ip add to my vlan 10 interface.

Can you help me out?

I am using 2960 switch on packet tracer