please advise your approach to break down a /20 IP subnet to /24 subnets

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Hi All,

Now im facing an issue. Currently in my environment, my core switch has a big flat VLAN of /20 subnet mask.

interface Vlan 1

ip address

That is to say, the range cover from .128.x till .143.x

Management would like to break them dwn to multiple class C subnets instead of a big Flat VLAN. making use of the opportunity of office expansion, i would like to set things right for a couple of new VLANs first before changing the whole IP addressing and configuration.

That is to say, for my new office area, i would like to use .133.x/24 segment.

Here is my question (i haven tried)

if on the core switch, am i allow to create the following config

interface vlan 133

ip address

knowing that this config conflicts with the previous VLAN configuration, will my Core switch allow to me set this IP address? since it interlap into /20 vlan.


I have this problem too.
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Edison Ortiz Thu, 05/28/2009 - 04:43

No, it won't allow you to bring up that SVI and you will receive an error message about 'overlap subnet'.

If you want to create new subnets with /24, then I recommend starting with subnet and onward -or- up to (if those subnets are available in your network).




Hi Edison,

thanks for your reply. thats what i thought so and its something i dont wish to do so as that IP addresses has been used up

i have came out a phases approach to address it and now im stuck with a problem. let me describe to you guys.


1) A redundant dual-core network

2) IP addressing/VLAn not greater than class C

N.b Currently, the environment is only running on a single core switch 6509

Stage 1:

Configure Core 2 (6506) with multiple class C vlans (.129, .130, .131, .132, .133) shut down all VLANs except 133 to support new office.

Connect Core 2 to Core 1(6509) using layer 3 point-to-point instead of Trunking. this allow co-existent of same IP addresses. Configure static route on Core 1 to route 133 traffics to Core 2, on Core 2 - default route to Core 1

Stage 2

Replace existing Core 1 (6509) to new Core 1 (6506), configure as trunk (layer 2) to Core 2 to form dual-core switching environment. VLANs should leran from Core 2 and now both switches will have same VLAN segments setup

either reboot access switches to re-learn the new VLANs info or wait for it to update itself. configure ports to correct VLAN.

Here are my questions:

in Stage 1:

the new VLAN 133 on Core 2 will not be able to get DHCP from the server connected on Core 1, users on Core 1 will not be able to "talk" to 133 on Core 2, as the network traffic will still be switch among it own subnet /20 ealthough there is a layer 3 static route on the msfc.

What should i do so that users on Core 2 Vlan 133 able to talk to servers on Core 1 VLAN.. cause my impression will be servers on Core 1 will reply traffic "moves" within its VLAN

On stage 2:

will the access switches able to re-learn automatically or need reboot?

Edison Ortiz Thu, 05/28/2009 - 05:34

On servers and workstations on stage 1 within the Core 1, you can manually add a route within their operating system. On Windows, you can use the 'route add' command - pointing to the Core 1 SVI.

Crazy enough, this may work - though I never seen nothing like it :)

On Stage 2, the access switches should be able to refresh their Vlan DB provided the VTP domain and password matches and they are configured as VTP Clients.




Joseph W. Doherty Mon, 06/01/2009 - 03:13

Have you considered using secondaries for the address migration? You'll want insure the address spaces don't overlap, but secondaries allow multiple (host) address spaces to share the same "wire". Once devices are on the replacement address space, you remove secondaries.

using the secondary ip method, may i know of they are feasible with such implementations

Core switch Vlan 1:

IP address

ip address secondary

ip address secondary

ip address secondary

ip address secondary

may i know if the above config will be ok? these IP overlap with the primary IP subnet.

next, with this config in place.

since all the users/printers are still in the vlan 1 (have not change yet), their DHCP IP address for e.g is Mask: Gateway:

Will they still work normally?

for subsequent new workstations/printers, i will configure them to use /24 mask and the respective gateway IP which i suppose it will work as well.

please advise,



Joseph W. Doherty Tue, 06/02/2009 - 04:35

When using secondaries, you still want to avoid overlapping addresses.

Secondaries lend themselves to dealing with static address hosts. You can go to each host, change its addressing, and it will continue to work.

Secondaries also can be used to allow DHCP hosts to migrate to new addresses as DHCP leases expire.

An issue you might have with DHCP hosts, is too many on an existing subnet to fit within a smaller subnet address space. You could go to much effort to control DHCP renewal per host, but if you can accept a brief network interruption, having a DHCP host see its link change state, I believe, will cause DHCP to confirm and, if necessary, obtain a new IP address.

I.e. for DHCP hosts, define the new subnet(s) on a different VLAN(s), and either repatch access connection, or shut access port, reassign port's VLAN, no shut port. (In other words, don't use secondaries.)


If you're tight on address space, you only need new to migrate to. Once subnet is migrated, you can reuse its address space.

Base on what you have said, i will not be able to configure this way. Because the main idea is to break down into multiple class C.

Any better suggestions?

Core switch Vlan 1:

IP address

ip address secondary

ip address secondary

ip address secondary

ip address secondary

Joseph W. Doherty Tue, 06/02/2009 - 09:57

Perhaps your missing the point that it's unlikely you can break down an existing active /20 into its component /24s. (For one reason, keep in mind that many /24s network addresses and broadcast addresses are /20 "host" addresses.) Think as if you were migrating to a new addressing scheme (which you are - just you'll be able to reuse most of the address space).

If you migrate the host addresses, in the existing active /20, into /24s not in any active address space, then both secondaries and/or new VLANs /24s could help the migration. Once you've "emptied" the /20, its /24s could be used for another active /20 (or you can move back into the original /20's /24s from the "new", and temporary, /24s).

BTW, if you do use secondaries and DHCP, I recall DHCP will only "see" the primary gateway address unless it's placed directly on the same subnet you want it to provide an address for. This often means the new address is configured as the primary and the old address as the secondary.


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