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overlapping ip - solved

paul amaral
Participant
Participant

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 

1 Accepted Solution

Accepted Solutions

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. 

View solution in original post

6 Replies 6

balaji.bandi
VIP Community Legend VIP Community Legend
VIP Community Legend

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

BB

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How to Ask The Cisco Community for Help

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 

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.

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. 

you are so so welcome friend

David Ruess
VIP Rising star VIP Rising star
VIP Rising star

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.

Subnets.PNG

Hope that helps

 

-David

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