Overlapping VLANs?

[Slideform]

Hi there,

I just got a new Cisco 350 for our network.
I'm trying to setup multiple VLANs that are able to talk to each other, but the status overview says something is wrong. I dont really understand why though. 

These are my VLANs.Since all our switches are inside the 192.168.250.0 segment, I gave the new one 192.168.250.130.
Once I configure its IPv4 interface, i get the following error:
%IPADTBL-E-IPSBNTOVRLP: A specified subnet 192.168.250.130 and mask 255.255.255.0 is overlapped with subnet 192.168.100.35 and mask 255.255.0.0.  

Why are the other two interfaces that are in the same subnet, not producing this error and is there a way to prevent this?

VLAN 2	Static	10.2.255.250	255.255.255.0	Valid
VLAN 3	Static	10.3.255.250	255.255.255.0	Valid
VLAN 1	DHCP	192.168.100.35	255.255.0.0	Valid
VLAN 250	Static	192.168.250.130	255.255.255.0	Valid

Best regards

[marce1000]

       >... with subnet 192.168.100.35 and mask 255.255.0.0.  
         - You need to set  that netmask to 255.255.255.0 too

 M.

[Flavio Miranda]

Hi

 With mask 255.255.0.0 and 192.168.100.x 

The next available network would be 192.168.101.x 255.255.0.0

If you want to use the network 192.168.250.0 you need to adjust the mask to 255.255.0.0

[Joseph W. Doherty]

"With mask 255.255.0.0 and 192.168.100.x 

The next available network would be 192.168.101.x 255.255.0.0"

Incorrect, I believe, because, the next available IP would be 192.169.0.0 as 192.168.100.35/16 is in the address block 192.168.0.0 though 192.168.255.255.

"If you want to use the network 192.168.250.0 you need to adjust the mask to 255.255.0.0"

Incorrect, I believe, because 192.168.250.0/16 is actually also network 192.168.0.0/16 and would conflict with the other VLAN using the same network.

[Joseph W. Doherty]

"Why are the other two interfaces that are in the same subnet, not producing this error and is there a way to prevent this?"

If you mean your two 10.x.x.x/24 networks, they don't overlap.

"I dont really understand why though."

Possibly because VLAN1 is using a /16 which overlaps with VLAN250's /24?  (The reason for the error.)

As I don't know what your address allocation policy desires or requires, cannot really say what the optimal correction should be, but very likely, @marce1000's suggestion to change VLAN1 to a /24 is what you might want really intend (it also should stop the overlap).

[Richard Burts]

The configuration of vlan 1 with mask 255.255.0.0 is a bit odd and claims all addresses in the range of 192.168.x.x. Vlans 2 and 3 are ok because they use addresses in 10.x.x.x. vlan 250 has a problem because it attempts to use addresses already reserved for vlan 1. For a solution you can either change the assignment for vlan 1 to use a smaller range of addresses, or you can choose a range for vlan 250 that uses something in 10.x.x.x or something in 172.16.x.x.

[Brandon Marr]

---

@Slideform wrote:

IPSBNTOVRLP: A specified subnet 192.168.250.130 and mask 255.255.255.0 is overlapped with subnet 192.168.100.35 and mask 255.255.0.0.  

Why are the other two interfaces that are in the same subnet, not producing this error and is there a way to prevent this?
Best regards

---

To help clarify the resolution that @Richard Burts provided, we can break it down using the error message...

A specified subnet 192.168.250.130 and mask 255.255.255.0 is overlapped....here we can see that the ip address 192.168.250.130/24 which is in the 192.168.250.0/24 subnet is "Overlapped" with another subnet.  The error then indicates to you the subnet that its overlapping which is 192.168.0.0/16, the culprit ip address here is 192.168.100.35/16.  

With that in mind we can see how they overlap because IP address' in the 192.168.250.0/24 subnet also apply to the 192.168.0.0/16

Truncated subnet outputs: BOLD text shows some of the overlap
 Subnet 192.168.250.0/24
   - 192.168.250.1
   - 192.168.250.99
   - 192.168.250.100
   - 192.168.250.101
   - 192.168.250.254
Subnet 192.168.0.0/16
   - 192.168.0.1
   - 192.168.250.99
   - 192.168.250.100
   - 192.168.250.101
   - 192.168.255.254

I hope this helps with your understanding

Disclaimer - only CCNA level network engineer here so please correct me if I am am incorrect at all.