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Subnet change, DHCP help

Posted on 2012-03-15
Medium Priority
Last Modified: 2012-03-19
I am in the process of changing our subnet from a /20 to a /19 to make way for a large influx (2000+) of new PC’s coming in.  All routers, firewalls, switches have been changed to the new subnet already.  My last step is to change servers and DHCP.  Currently we run a network, to are static, and plenty of room for static's.  DHCP pool starts at to and is nearing capacity.  

As for DHCP, what would be the best approach to add more addresses to the DHCP pool with the new subnet.  I’ve read a few things:
1. Doing a 80/20 split with another server and set a delay on the second one to prioritize
2.Delete the current scope and recreate with a wider address pool
3. Keep the current scope and create a super scope

I’m leaning towards just deleting the current scope and recreating with a to address pool.  Thoughts?  Do I need 2 DHCP servers?
Question by:rexnetworking
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LVL 11

Expert Comment

ID: 37725728

I would consider deleting the current scope and recreating it based upon your new IP address pool.  If you go with 2 dhcp servers, there is potential for conflict.
LVL 26

Expert Comment

by:Fred Marshall
ID: 37726707
Some would say that you have too many hosts as it is.  Doubling from there seems risky from a traffic point of view.  Only you can know but these are really big numbers!!

But, strictly from a technical point of view, I see no reason to not simply change the subnet mask and add an appropriate number of DHCP addresses at the DHCP server setup.
So, if you have:  /
and give the DHCP server 6400 addresses starting at then that seems consistent with the approach you intend to follow.

But, one might observe:
You currently have around 1279 static addresses available and that's "OK".
You currently have around 2815 dynamic addresses available and that's not "OK".
Do you really have that many hosts?  Or is maybe your DHCP pool being overtaken by lease times that are too long and lots of transient device activity?  If the latter, I would set the lease time at 8 hours or maybe even less if the transient activity is higher than 1-time each day appearances.
Anyway, you plan to end up with a total of 6900 dyamic addresses???  wow.

You probably will want to split things up into subnets that are smaller and route between them.  But let's make sure the requirements are clear first.  Just how many simultaneous hosts are operating?

Author Comment

ID: 37727470
There will be a lot of hosts, we currently have around 2500, and will be adding another 2500 in the coming months, We will be going 1 to1 with employees and laptops in the coming months on top of what we currently have.  We'll be around 5k dhcp hosts when its all said and done.  

So i was just trying to evaluate the best route to take, if 1 DHCP server will  handle that or if I am better off splitting it up between 2 to load balance.
LVL 26

Accepted Solution

Fred Marshall earned 2000 total points
ID: 37730320
With these numbers I would be *much* more concerned about network traffic than about DHCP server load.  The amount of DHCP traffic is pretty low.  It's going to be something like:

Host asks for an address lease.
Server provides an address lease. ..... done

Host waits for half the lease time (measure in hours or days) .. no traffic involved here.
Host asks for a lease renewal.
Server renews lease. ..... done

The DHCP server will have to be running some sort of service or daemon that will check the lease times and terminate them when they expire.  But that can't be much computing load I wouldn't think.

As before, I would suggest short lease times if you have a fair amount of transient users.
The tradeoff might be if all the computers are turned on in the morning at the same time and, because of short lease times, have to renew their leases all at once then *maybe* there would be a bit of a traffic jam.   I have no idea.  I suspect this concern is overkill.  Anyway, for this reason you might make the lease times longer and that will almost certainly guarantee that they will renew out of sync.

How much transient user movement is there out of the 2500 kinds of numbers?

This perceived problem resolves itself if you split the network up into multiple subnets because you'd likely have separate DHCP servers.  Then you deal with the broadcast traffic issue, which is more the real issue, at the same time.

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