Vlans and subnets

[WILLIAM STEGMAN]

I have a /21 network and a corresponding vlan for that network.  Could I expect to create a 2nd vlan and corresponding subnet within the /21 supernet on the same switch without issue?  For example

10.1.1.0/21 is vlan 10

10.1.2.0/24 is vlan 20

I understand I would probably not want to place any hosts on the 10.1.2.0/21 network.  This seems possible, but just doesn't look right to me.  

Thank you

[Peter Paluch · Accepted solution]

Hi William,

No, this is not possible. A multilayer switch will not allow you to configure overlapping IP networks on its SVIs. Once an IP network is directly connected to a particular interface, it is considered to be entirely and completely present on that interface. It would be illegal to state that out of a sudden, an arbitrary portion of it is located on a different interface.

You will probably need to subnet the original network into smaller parts if that is possible, and use the subnets in individual VLANs.

Just a remark - your address example is not entirely right. 10.1.1.0/21 is not a valid network address. 10.1.0.0/21 would be a valid network with a /21 netmask, 10.1.8.0/21 would be another valid network, but 10.1.1.0/21 is a host address within the range 10.1.0.0 - 10.1.7.255.

Best regards,
Peter