Overlapping addresses

[Frank Franciamore]

Hello!

I am playing around with a lab and I came across an error in my configuration:

Gi0/0 is configured with IP address 192.168.7.27/24

Gi0/1 is configured with IP address 192.168.7.47/27

The error that pops up is "192.168.7.32 overlaps with Gi0/0".

If I change Gi0/0 to have a /27 mask, it resolves the issue - both interfaces will have the same subnet mask, their IP addresses will be in different subnets, and there will not be any duplicate IP addresses between those subnets. Thus, no overlap. But why can't I change the Gi0/1 to have a /24 mask? I still get an overlap error, but I do not understand why. Intuitively, I assumed that changing Gi0/1 to a /24 mask would put everything under one network between both interfaces and clear them from having overlapping IP addresses (as each interface can only be assigned 1 of the 254 host addresses for that subnet mask), but that's not the case. Can anyone explain why?

[Georg Pauwen · Accepted solution]

Hello,

if you assign a /24 mask to interface GigabitEthernet0/1, it will be in the same subnet as GigabitEthernet0/0. You cannot have two interfaces in the same subnet.

[Frank Franciamore · Accepted solution]

Thank you for the reply. Now I understand why my intuitive assumptions were wrong. 

Originally, I was under the impression that if both interfaces shared the same /24 subnet and Gi0/0 was using the 192.168.7.27 address, then Gi0/1 will not be able to use 192.168.7.27 because it was taken by Gi0/0. Therefore, Gi0/1 would need to be assigned a different IP address. This is not the case...Amateur mistake. 

Now I understand that interfaces must be mutually exclusive in their configurations. Two interfaces means two different subnets. Of course, there are ways around this notion as others have stated, but for basic networking theory - the standard is to have interfaces be in different subnets. 

Thank you for the clarification. 

[Martin L]

Each router interface must be in different network/subnet.

Practice subnetting a lot. find video and method that you understand; most likely decimal way of doing things using magic number and not binary subnetting (although good for advanced summarization).
Cisco ccna exam will ask u to do subnetting very fast.

if u are not on CLN, go there and find posts about sunbetting. https://learningnetwork.cisco.com/content?filterID=all~objecttype~objecttype%5Bthread%5D&itemView=detail

[paul driver]

Hello

As stated by others you cannot have two interfaces on the same rtr in the same subnetwork range - However if those same interfaces are each in their own VRF instance then they can have the same subnet mask and even have the same ip address.