overlapping ip - solved

[paul amaral]

Hi, originally we were given a /27 for the WAN on a metro circuit. I was hoping to extend this without buying more routable ips for the LAN. So I took the /27 and divided it, creating new subnets. The /27 was divided to a /30 for the WAN and a /28, /29 and /30 for other interfaces. The issue is when I try to configure the /28 to a different interface, when the /30 is already configured on the WAN interface, I get an overlap error. If I secondary these subnets on the same interface the overlapping goes away but I was hoping that there could be other workaround, besides NAT, that I can use. This is on IOS XE 16.9.5. 

Solution: UPDATE, looks like I have to divide the subnet via boundary lines, either all /29's or two /28's.

What happened was that I was taking the original /27 and subnetting it into a /30 1st then a /28, /29 and /30. This would cause the 1st /28 to overlap with the /30. What I should of done originally is break the /27 into 2 /28's and break those down further in consecutive order following the subnet boundary lines. 

Thanks, P 

[balaji.bandi]

You need to give example what IP it is overlapping : example 192.168.1.0/27 - /29 below

192.168.1.0/28
192.168.1.16/28

=================

192.168.1.0/29
192.168.1.8/29
192.168.1.16/29
192.168.1.24/29

=============

/30 as below

192.168.1.0/30
192.168.1.4/30
192.168.1.8/30
192.168.1.12/30
192.168.1.16/30
192.168.1.20/30
192.168.1.24/30
192.168.1.28/30

[paul amaral]

Hi, for example if I originally have a /27 and break it to one /30 and one /28, 

ip address 161.x.x.162 255.255.255.252 on gig0/0/0 (161.x.x.160/30) 

then add, 

ip address 161.x.x.165 255.255.255.240 on gig0/0/1 (161.x.x.164/28) 

I will get an overlap error. However if I break the original /27 into two /28's there's no overlapping error. This is within the rules of VLSM so not sure why the router gives me a hard time about the /30 160-163 and then configuring the /28 164 - 179

Paul 

[MHM Cisco World]

friend what try to explain here,
for example I divide the /24 into two large daughter subnet,
then I use one of this daughter subnet and child of other daughter subnet there will no any overlap.

here you divide /27 into two /28
you use one /28 in interface and YOU can use /30 of other /28 (child) in other interface there is no overlap at all.

[paul amaral · Accepted solution]

Hi, what you suggesting does work, what was happening was I was taking the original /27 and subnetting it into a /30 1st then a /28, /29 and /30. This would cause the /28 to overlap after I assigned the /30. What I should of done originally is break the /27 into 2 /28's and break those down further in consecutive order. 

[MHM Cisco World]

you are so so welcome friend

[David Ruess]

Hello,

This may help with your ranges of IPs that don't overlap.

https://www.davidc.net/sites/default/subnets/subnets.html

You can keep clicking divide to get your subnets.

Hope that helps

-David