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Recommended size of Subnet

Posted on 2014-10-25
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Last Modified: 2014-10-26
I wonder what's the recommended size of each subnet. for instance how many hosts should be in each subnets, considering mapping Vlan to subnet.
Example:
let's say I have this network 172.16.0.0/16 that I will subnet and make it 172.16.0.0/24
This will give me 256 Subnets of 254 hosts each, and each Subnet will have it is own Broadcast address : example:
Network:   172.16.0.0/24          
Broadcast: 172.16.0.255          
HostMin:   172.16.0.1            
HostMax:   172.16.0.254          
Hosts/Net: 254                    


Network:   172.16.1.0/24          
Broadcast: 172.16.1.255          
HostMin:   172.16.1.1            
HostMax:   172.16.1.254          
Hosts/Net: 254

As know Best practice is to map each subnet to a specific Vlan. however I am not sure if 254 hosts in one  Vlan does not impact performance.

OR do I need to subnet it further:  to /25 or /26 in order to have smaller subnets ?

Thanks
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Question by:jskfan
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Don Johnston earned 860 total points
ID: 40404354
Unfortunately, there is no absolute answer.  It used to be the size of the network was limited by collisions. But now that we use switches, collisions are no longer a limiting factor.

Excessive broadcast can be a limiting factor (rule of thumb is more than 20% is excessive). But you have to have a lot of devices to hit that number.

So at the end of the day, the size of the subnet usually becomes a management/security issue.  Which means as large as a few thousand to as small as 2.
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Assisted Solution

by:Akinsd
Akinsd earned 860 total points
ID: 40404372
Avoid subnets larger than 510 hosts (/23) as much as possible
Cisco still recommends a maximum of 500 which is 510
I also recommended also to have QoS in place, but a MUST for networks with more than 500 hosts (all subnets combined) to ensure that 1 host does not hog traffic unecessarily.

/25 (126 hosts) or /26 (62 hosts) are good as long as you have enough IPs for hosts in respective subnets.

Broadcast storms may cause congestion, collisions and eventual packet loss. Mac Address flooding can cause a switch to function as a hub and will pass traffic in all directions.
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Expert Comment

by:Fred Marshall
ID: 40404384
I agree with Don Johnson.  One size does not fit all.  That answers your question pretty well.
It's important to first define what your problem is.
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Author Comment

by:jskfan
ID: 40404429
172.16.0.0/25 will give 512 Subnets with 126 Hosts each.

So if a VLAN has 126 hosts , will that be too much with regards to performance ?
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Assisted Solution

by:Fred Marshall
Fred Marshall earned 280 total points
ID: 40404463
As I said in other words: "it depends".

With 126 hosts AND smart switches AND typical office traffic then most certainly.
Without the smart switches - very likely yes.
But, if the traffic is unusual then ... how unusual is it?
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Expert Comment

by:Akinsd
ID: 40404479
You should be fine with /25.
2 subnets with 126 hosts each constitute 2 separate broadcast domains as they have a routed interface separating them. That's one of the main purpose of subnetting, to contain broadcast domains in small chunks.
It's like dividing a class of 510 grade 2 students into 2 classes. Imagine 126 students shouting (broadcasting) messages to everyone compared to 510 students shouting messages. There is more discombobulation in the class of 510 than 126.
You should however implement QoS on your core switch. Auto QoS should suffice
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Author Closing Comment

by:jskfan
ID: 40404976
Thank you
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