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Whats the rule of thumb on the amount of hosts per network segment?

Posted on 2011-09-09
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My network consists of 3 different networks (VLANs).  All three VLANS serve a different purpose.  When I inherited this network I sort of laughed at how big the subnets were for each VLAN:

VLAN 30 - 10.4.0.0/16
VLAN 40 - 10.5.0.0/16
VLAN 50 - 10.6.0.0/16

These are very large networks.  I was always told that the rule of thumb is to never make a network segment larger that about 250 hosts or a Class C network, or broadcast traffic could start to degrade performance.   VLAN 30 is about to reach 230 hosts.  Most of these hosts are servers that host applications.  Should I created another VLAN and start adding new servers to this network.  I'm worried that VLAN 30 will get to congested if I add to many more servers.
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Question by:denver218
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jmeggers earned 500 total points
ID: 36511775
In todays switching world, there's a lot less broadcast traffic than we used to see with hubs, so I think the limit is higher than it used to be.  That said, I wouldn't recommend using all of your /16s.  ;-)   But I wouldn't be too concerned about your existing VLAN 30 for now, and even if you add another 20 or 30 servers to that VLAN, you should be fine.  

At some point, you're going to have to make a decision about starting a new VLAN, though, and your question is really where is that point.  I doubt if you'll find a specific number anyplace.  My suggestion would be to monitor your network for collisions, CPU loads on the switches, any indications performance is starting to be affected, and be prepared to start to segment the network when you start to see those kinds of problems.
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by:denver218
ID: 36512046
Thanks.  My switching network looks as follows except I have 5 switches on VLAN 30.
 SwitchesVLAN 30 has five 3550 switches all connected together via fiber (GBIC Ports) and then one of the switches has a link to my 3550-12G Core Switch.  Do you think if I add a 6th switch on VLAN 30, I should go straight into the core switch with this one, instead on connecting it to one of the other 5 switches?
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by:eeRoot
eeRoot earned 500 total points
ID: 36513279
Older network equipment with lower end CPU's might have trouble with a setup like this, but newer equipment should be able to handle it.  If you can, it would be good to have a monitoring tool watching the CPU utilization on the switches.  If it's past 75%, you may start to see problems like dropped packets or slowed connections.
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by:Mizzio59
Mizzio59 earned 500 total points
ID: 36514163
From the picture it seems that VLAN20 is backbone Vlan and the VLAN 30, 40, 50 are edge/users Vlan and are not able to talk between each other if not roted thru Vlan 20.
If you connect an additional switch for vlan 30 directly into Core Switch, you create an isolated part of vlan 30, because users are not able to reach default gateway the is located in another attached switch that use backbone vlan to connect.
So it is easier to  connect it to one of the 5 switches dedicated to Vlan30.
About CPU utilization, we have a vlan made by 5 (or 6) x 48 ports < 300 users(mac address)... not really to much;
Spanning tree protocol instance is limited by the backbone vlan that separates each vlan... ( STP instance for VLAN30 is spread to 6 switches)
I think you can go safe.
ciao,Maurizio
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by:bgilsing
bgilsing earned 500 total points
ID: 36519509
I agree with eeroot that you should haves plan ready to segment and base you decision on performance. With the equipment you have you can surely expand to over 300 hosts on a single subnet. Use a network monitoring system such as nagios to check you stats on the switches and when your getting close pull the trigger on segmenting. You will probably find that you need to add equipment as the number of hosts will be the same and the CPU needs to deal with them regardless of how many LANs you have. I would link everything through the core switch.
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by:denver218
ID: 36522161
Thanks everyone.  I use Solarwinds Orion for Network Monitoring so I will just keep an eye on performance then.  Currently my CPU usage on the VLAN 30 switches are only at 3% and memory is at 24% so I believe I will just keep adding devices until I see a performance issue.  Thanks.
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