Number of Devices per VLAN

[ALIAOF_]

Is there a best parctice, white paper, guide etc on how many devices can be in a specific VLAN?

[Jon Marshall · Accepted solution]

Mohammad

It is application specific to be honest. If you have a lot of apps that use broadcasts then you may want to use a smaller subnet. 

If you do a quick search through these forums you will see there is not necessarily agreement but the general concensus seems to be no more than 2 class C's ie. 512 hosts.

Personally i think that is a bit big If you go with a /24 per vlan that would be a good start. Personally i have come across  /24s and /25s in the networks i have worked on.

Jon

[Joseph W. Doherty · Accepted solution]

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Posting

Often in a switched environment a 254 is fine, but whether you can size larger or need to size smaller depends on the kind of traffic and how much of different kinds.

The primary determinate for choosing the number of hosts would be how much broadcast traffic will be present.

The secondary determinate would be how much multicast traffic will be present if there's no multicast suppression (e.g. IGMP snooping).  With multicast, you might also need to account for multiple L2 multicast mapping to ranges of L3 multicast.

Lastly, if yours is a shared media environment, you'll need to account for how much overall traffic there will be.

[ALIAOF_]

Thank you for the reply every one.  So basically 254 to 512 is what we are looking at depending on the applications etc.  Now we have a network for instance with approximately 900+ devices, how often something like that you have encountered?  I guess I can put wireshark on it and see how much broadcast traffic is showing up as a justification to decrease the size, any thoughts?

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Most of my experience has been using /24s within large L3 topologies.  In situations where we could have used a /23 or even a /22, since /24 generally works fine, why risk going too large.  Conversely, where a /24 wasn't really needed, I've often used it if using private 10.x.x.x addressing; avoids the need to later expand an address block if allocated a little too tightly. (On this later point, often worthwhile to avoid completely filling a /24 too.)

Overall, for routine network applications, you'll probably be fine using a /23, /24 or /25.  If there's something usual about your network applications or other considerations pushing for smaller or larger address blocks, then you might want to get into additional analysis such as sniffing an existing subnet to analyze its traffic.