can 2 vlans have the same subnet?

[jpflory01]

I hope the combined genius of the fellow community can answer me this. I am new to Cisco, and I understand VLANs as a physical boundary separating broadcast domains.

I was wondering if it is possible to divide 1 subnet (192.168.1.0) into two separate VLANS? I have all layer 3 switches in my environment. Making matters worse, there would be no pattern for the IP address assignments into VLAN-A vs. VLAN-B..

If this is possible, can you please explain the mechanisms for a successful implementation. 

[uniemeyer]

Hi, unfotunately it is alittle bit unclear why you like to do it! In most of the cases you would create two subnetworks e.g. going from a/24 network to two /25 networks and setting up the DHCP scope accordingly! 

However if the two networks dont need totalk to each other, you could also use the vrf light feature, but your layer 3 switches must support that feature.

[Marvin Rhoads · Accepted solution]

It mostly depends if/how you want hosts on them to talk one another (or other networks).

If the answer is "not at all" then you can have as many VLANs as you like using the same subnet. 

If the answer is "completely" then you have to either a. break your addressing (L3) down to have one set of hosts in subnet A (on vlan a) and the others in subnet b (on VLAN b). or b. have some fancy tricks in place with network address translation (NAT) in place.

I'll leave the latter solution off as beyond the scope of your question.

For the former, you would just change your subnet mask - for example, if the classful subnet is a "standard" /24 (255.255.255.0) then split it in two - /25 or 255.255.255.128. Assign hosts in one or the other.

You have to have some pattern - all networking is based on patterns in some way or another.

[jpflory01]

Thank you both for your answers, 1 year later, I understand everything you say, and realize how unorganized my network is, and what I will need to straighten it out. Sorry for being late to the party, but thank you for your answers.