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How many nodes are acceptable on a vlan?

bpl5000
bpl5000 asked
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Last Modified: 2012-05-06
We have several vlans... one for each building.  One building is using up almost all available IP addresses.  Currently the subnet mask is 255.255.254.0 so we have a total of 508 usable addresses.  We have 2 options, add another vlan to the building or change the subnet mask to allow more IP addresses.

For manageability, it would be preferable for us to change the subnet mask and up the number of available hosts on the subnet.  The thing I'm wondering is what number of nodes would be acceptable to have on a vlan?  We have a Cisco 4006 and I want to make sure we do not impede performance.
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Commented:
There's no limit to the number of nodes on a VLAN.  The only limitation will be the number of IP addresses available for you to use.  You will just need to make sure your routers have the ability to communicate between the VLANs.

Think of a VLAN as a LAN, as it is just a virtual LAN.  The only limitation to a LAN is the number of available IP addresses.  I will say though, if you have 1,000 hosts, I wouldn't recommend using a home router to handle the routing! ;-)

Moral of the story, the speed of your hardware will determine how many nodes your network equipment can handle, but that doesn't have anything to do with VLANs.
Top Expert 2009

Commented:
Just to add as a second bit or reassurance.

Changing the mask is fine.  We use /22 masks in our buildings.
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Commented:
There is no limit on nodes, but do be aware that large broadcast domains will slow a network down.

I would break it up into domains as small as logically possible.

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Commented:
Piattnd, like I mentioned, we have a Cisco 4006, not a home router.  I was looking for people with Cisco experience (I should have mention that).  We will have 700+ devices and maybe up to 1,000 someday.  I know PC usage and things like that make a difference too, but I'm wondering if there is any general type of guideline for how many nodes should be limited to a vlan.

We are strictly using TCP/IP.  No servers or databases are contained within this vlan.

JFrederick, approx. how many nodes do you have in your environment?
Top Expert 2009
Commented:
Best practice would be to have /24 subnets and route between them.  With hardware today and non-broadcast apps, there really is no performance/useability downside to routing between subnets.  The issues come in with management like you said (DHCP reservations, moves, IP filters/restrictions, etc...).  It would be simpler to have one large subnet but its cleaner and more desirable to have smaller subnets/broadcast domains.  It's a tradeoff between performance (NIC's processing lots of ARP traffic) and ease of management.

If you are truly planning to have 1000 devices in one VLAN, I would take this opportunity to break it out into multiple VLANs.
Don JohnstonInstructor
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Top Expert 2015

Commented:
The rule of thumb for VLAN size is no more than 20% broadcast traffic. So as long as the broadcast don't exceed 20% of the total traffic on the VLAN, it's not too big.


Author

Commented:
I read something similar that stated the 20% broadcast rule when determining size, but to me that makes no sense.  For example, let's say I have a subnet of 255 hosts with 20% broadcast traffic, then I double the subnet to 510.  If the 255 hosts that I added broadcast very little, but create a lot of other traffic, then the broadcast percentage would go down even though the vlan size has decreased.

I contacted Cisco and the engineer's recommendation was this...
-Up to 500 hosts if speakers are only using TCP/IP and fairly quiet
-Up to 300 hosts if using Suns, or NetBIOS over TCP on PC's
-Up to 200 hosts if using AppleTalk or DECnet

Of course there is no rule written in stone since it depends on variables that will differ from one network to another.  However, I believe JFrederick is correct when he says that smaller is better when considering performance.  We have now decided to keep the subnets at the current size and just add an additional subnet.

Thanks for the help!
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