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overlapping errors

If I have a network setup with an ip address schema of 10.1.x.x /16 and I decide to create vlans on 4500 series switch to make best use of my ip addresses, it will give me an overlapping error with vlan 1when I configured my additional vlans ip address as follow:

interface vlan 1: 10.1.0.1 /16

interface vlan 2: 10.1.2.1 /23

interface vlan 3: 10.1.4.1 /23

I thought that since the subnet mask assigned a different network id to vlan 2-3, vlan 1 should not care. Is the error caused by the fact that an ip address in vlan 1 could potentially be found in vlan 2 or 3 regardless of the subnet mask? Is it a bad design practice to do so? If not, is there a way go around this problem?

I am using a 4507R switch with some 3500 series cisco switches and some 3900 Coms

Thanks for the help

1 ACCEPTED SOLUTION

Accepted Solutions
Gold

Re: overlapping errors

If you have to stay inside the 10.1.x.x numbering plan, then change the mask on interface vlan 1 to 10.1.0.1 /23. You will also need to change the mask on any VLAN 1 devices to match; if you don't, the VLAN 1 users will have no problem talking to other machines on VLAN 1 with 10.1.0.x and 10.1.1.x addresses, but will not be able to respond properly back to 10.1.2.x and 10.1.3.x users on VLAN 2, or to 10.1.4.x and 10.1.5.x users on VLAN 3.

If you can expand beyond 10.1.x.x, save yourself the trouble of changing all those masks and leave VLAN 1 as a /16; and just start numbering your /23 subnets from 10.2.x.x: for example, 10.2.0.1 /23, 10.2.2.1 /23, 10.2.4.1 /23, etc.

It is a bad idea to knowingly create an overlapping network address situation, if it can be avoided. Routers and L3 switches like to know precisely where to deliver a unicast packet to, and overlaps confuse the issue.

That being said, there are situations where identically-numbered networks need to be connected, yet keep their numbering. You can do this with NAT, but it's more complicated than just readdressing/masking the subnet(s) with the smallest number of users affected (as I have outlined above). In case you're interested, though, here's a link:

IP addressing Services: Using NAT in Overlapping Networks

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_configuration_example09186a0080093f30.shtml

Hope this helps.

2 REPLIES

Re: overlapping errors

Its always a good design criteria, to assign non overlapping subnets in your network. As long as the ip addresses assigned to workstations/servers in vlan 1, 2, 3 doesnt overlap it should work fine.

Its a better practice to do variable length subnetting, and assign a subnet mask to each vlan, depending on how many hosts are needed on that vlan.

Say if you need only 12 hosts in a vlan, its better to assign a /28 mask for the subnet, which can support upto 14 hosts. When you need only 2 hosts in a subnet (wan point to point links), assign a mask of /30 and so on.

Hope that helps

Gold

Re: overlapping errors

If you have to stay inside the 10.1.x.x numbering plan, then change the mask on interface vlan 1 to 10.1.0.1 /23. You will also need to change the mask on any VLAN 1 devices to match; if you don't, the VLAN 1 users will have no problem talking to other machines on VLAN 1 with 10.1.0.x and 10.1.1.x addresses, but will not be able to respond properly back to 10.1.2.x and 10.1.3.x users on VLAN 2, or to 10.1.4.x and 10.1.5.x users on VLAN 3.

If you can expand beyond 10.1.x.x, save yourself the trouble of changing all those masks and leave VLAN 1 as a /16; and just start numbering your /23 subnets from 10.2.x.x: for example, 10.2.0.1 /23, 10.2.2.1 /23, 10.2.4.1 /23, etc.

It is a bad idea to knowingly create an overlapping network address situation, if it can be avoided. Routers and L3 switches like to know precisely where to deliver a unicast packet to, and overlaps confuse the issue.

That being said, there are situations where identically-numbered networks need to be connected, yet keep their numbering. You can do this with NAT, but it's more complicated than just readdressing/masking the subnet(s) with the smallest number of users affected (as I have outlined above). In case you're interested, though, here's a link:

IP addressing Services: Using NAT in Overlapping Networks

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_configuration_example09186a0080093f30.shtml

Hope this helps.

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