RE: Increase DHCP numbers
From: S.J.Haribabu (sjhari_at_microsoft.com)
Date: 08/06/04
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Date: Fri, 06 Aug 2004 19:13:22 GMT
Hi Gerry,
If your scope already covers the entire range and is fully used, you only
have two other options: superscoping or resubnetting. Both of these options
require you to make architectural changes to your network.
Simply changing the DHCP scope parameters does not give you more leases.
DHCP runs on top of your network subnet architecture and can hand out
addresses however you want. Always treat the need to expand address ranges
as a subnet architecture exercise first and foremost. After you decide
which architecture to use, you can configure DHCP to conform to your
network design.
Resubnetting
Resubnetting is the recommended procedure for increasing a DHCP scope when
the current scope has entirely consumed the current subnet mask. However,
this method requires you to change all subnet hosts and gateways. If you
have an address range that has run out of available host addresses, you may
be able to change the subnet mask to include a larger share of host
addresses. But simply changing the subnet mask requires that all routers
and other statically assigned computers be reconfigured and all DHCP
clients have renewed their lease obtaining the new parameters.
Additionally, the entire DHCP scope or scopes must first be deleted and
then re-created using the new subnet mask. The potential for duplicate
addresses exists during this period if you do not take steps to prevent
leasing addresses that other clients may use. Despite all of the
aforementioned caveats, resubnetting is still the recommended procedure.
The resubnetting configuration creates no additional overhead on the subnet
routers or gateways, and keeps all host on the same broadcast address.
The following example shows a depleted subnet with the following settings:
Subnet Address: 192.168.1.0
Subnet Mask: 255.255.255.0
This yields a network of 254 hosts with addresses from 192.168.1.1 to
1921.68.1.254.
The following example shows the result if you use the resubnetting option:
Subnet Address: 192.168.1.0
Subnet Mask: 255.255.254.0
You now have a network of 510 hosts with addresses from 192.168.0.1 to
192.168.1.254 (for scope 192.168.0.0), or 256 newly available DHCP
addresses.
Before:
---------192.168.1.0/24-------R-------192.168.5.0/24---------
After:
---------192.168.0.0/23-------R-------192.168.5.0/24---------
Superscoping
Superscoping (also referred to as multinetting) may meet your requirements.
If you do not want to change the subnetting of an existing network, you can
add additional logical networks to the same physical wire. This puts an
additional burden on the router or gateway configured with multiple logical
subnets running on a single physical port. The additional burden may result
in reduced network performance. Hosts on one logical subnet must be routed
through the gateway to communicate with hosts on the other logical subnet,
despite sharing the same physical wire.
The following example shows a depleted subnet with the following settings:
Subnet Address: 192.168.1.0
Subnet Mask: 255.255.255.0
The following example shows the results if you use the superscoping option:
Subnet Address: 192.168.1.0 and 192.168.2.0
Subnet Mask: 255.255.255.0
You now have two networks of 254 hosts (508 hosts total) with addresses
from 192.168.1.1 to 192.168.1.254 and 192.168.2.1 to 192.168.2.254, or 254
newly available DHCP addresses.
Before:
-----192.168.1.0/24------R-----192.168.5.0/24--------
After:
-----192.168.1.0/24 and 192.168.2.0/24-----R-----192.168.5.0/24------
After you decide which option you want to use, you can choose the
corresponding DHCP configuration.
If you use the resubnetting option, you need to delete and re-create the
DHCP scope with the new subnet mask (it is not possible to change only the
mask for a particular scope). If you are servicing existing clients within
a portion of this range, you should turn on conflict detection until all
your clients are migrated into the new scope. This action requires you to
perform the following steps:
Configure the interface of each connected router and change the IP address
for the connected interface, its subnet address, and its subnet mask.
Delete your current DHCP scope.
Create a new DHCP scope with the new subnet mask.
Enable the Conflict Retries option on the DHCP server (set to 1 or 2).
Force your DHCP clients to renew their DHCP leases.
Change the IP address, subnet mask, and/or default gateway on each
statically-configured host.
When you use the superscoping option, you need to superscope a number of
scopes together. Create each scope individually and then create a
superscope to incorporate the individual scopes. This action requires you
to perform the following steps:
Add secondary IP addresses to the current router interfaces.
Create a new DHCP scope for the new logical subnet.
Create a superscope and add the old and new DHCP scopes as children.
For more information look into http://support.microsoft.com/?kbid=255999
Gerry, Please let me know whether the information is sufficient for you.
Thanks,
sjhari@online.microsoft.com
This posting is provided "AS IS" with no warranties, and confers no rights.
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