In a Private LAN why use variable length subnetmask?

Posted on 2009-03-29
Last Modified: 2012-05-06
If the recommended number of nodes in a subnet is no more than 512. Why do people mess with the subnetmask to create variable length subnetmasks (private IP's)?

Why not use multiple subnets as below and stick routers in between

With standard subnetmasks you can look at the IP and subnetmask and visually know if two addresses are on the same network. But when using variable length subnetmasks you need to do the working out.

I understand it's useful with public IP's, There must be an advantage in private LANs

Would appreciate someones help

Question by:pancho15
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Expert Comment

ID: 24015074
The only real advantage to using VLSM in a network with private IPs would be in route summarization. However: to most network engineers using a /24 subnet for a point to point router connection would bother them on a fundamental level, even if they have the IPs to do it.

Author Comment

ID: 24015234
thanks Sniper,
So your saying you would use a VLSM on the interfaces connecting the routers?

So if you had 10 interfaces connecting routers you would use VLSM to create a suitable IP range ?


Accepted Solution

Sniper98G earned 500 total points
ID: 24015964
I do.

My network uses primarily Public IPs so I am always very conscious of my IP usage. I have a layer 3 dual core with 14 distribution points all connected to each core using point to point router connections. If I used /24s for all of those I would be wasting 7000 addresses.

Author Closing Comment

ID: 31564120
Thanks you for your help -  makes sense

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