Help breaking up a flat network

Unanswered Question
Sep 27th, 2009

I inherited this network that I think is way too flat. They only have 2 vlans (users and Servers). I am not concerned with the users vlan, but the Servers vlan is way too flat. I am trying to break it up without causing major issues (also , trying to keep it so I don't need to hit every server to change mask, etc.)

The Server vlan layer 3 interface is configured with /16. When they were assigning IP to the various Servers, Printers, etc.. on this vlan, they actually used a decent numbering scheme, but unfortunately, it's all on the same vlan using /16 for the mask

For example: for server, for printers, etc.. However, all have /16 mask and all on the same vlan.

Any ideas how to incorporate more VLAN's without destroying the whole environment? Another question, If I reconfigure the layer 3 interface with /24 subnet, would a machine with /16 subnet still communicate with the gateway?


I have this problem too.
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glen.grant Sun, 09/27/2009 - 16:16

If you want it to work right then you will have to change all the masks to a /24 otherwise leave it like it is . I don't know why people use like a /16 must be a complete lack of undertanding of addressing and masking .Make a project out of it , don't know how many total devices you have but to do it right you need to break it up into /24's ,make /24 SVI's on your layer 3 switch , put the ports in the right vlans and then address and mask the devices correctly.

Giuseppe Larosa Mon, 09/28/2009 - 05:48

Hello Imad,

>> If I reconfigure the layer 3 interface with /24 subnet, would a machine with /16 subnet still communicate with the gateway?

yes if you have proxy-arp enabled on the L3 interface

you can check if proxy-arp is enabled by using

sh ip interface type x/y

looking at the Proxy ARP line to see if it is enabled.

the device with /16 mask will ARP for an ip address it considers in same subnet: L3 interface of network device receives it and answers with its own MAC address in an effort to help the device with /16 mask.

Actually you need to increase the number of L3 interfaces and of L2 broadcast domains.

Hope to help


xcz504d1114 Mon, 09/28/2009 - 06:12

I inherited a decent sized infrastructure a year and a half ago with the same issues, they assigned /16's everywhere, in fact if they didn't have 70 remote sites T1 connected they would have had a giant /16 for everything I'm sure.

The only place I didn't break up the /16 was in my datacenter, the complications with trying to migrate the IP address of a Domain controller on a production network generating several billions a year was not something I wanted to do, so I actually built my network around the Datacenter. Luckily for me they used, the 64 makes an excellent boundary for summarization.

Honestly, most places don't have thousands of servers, leaving your servers in a /16 probably isn't that big of a deal unless you do actually have thousands of servers. The biggest thing is to reduce the size of your broadcast domains, since you probably have more users than servers, it would make sense to actually start there. Breaking up the voice and data networks is a great start, logically defining VLAN boundaries (IE IT data and voice VLAN's, HQ Data and Voice VLAN's) you can use physical buildings as VLAN boundaries, departments, whatever you would like, so long as you pick a standard, document it, and enforce it.

HTH get you started,



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