Best Practices for IPAM subnet containing DHCP Scope not at CIDR boundary

Question

I am rebuilding the entire IPAM structure to include all of our subnets and DHCP scopes. I am having trouble understanding how IPAM maintains IPs for subnets that are larger than the DHCP scope address pool and handles excluded addresses in the DHCP scope. I am in the unique position of adjusting either IPAM supernets/subnets and/or DHCP scopes. What is the best practice?

An example will illustrate best the questions.

Current Situation:

Subnet is 10.33.8.0/21

DHCP distribution range: 10.33.11.1 --> 10.33.12.100

excluded addresses: None

Options, which is it better?

1. Simple, current: Does this work well?

Configure IPAM with a subnet 10.33.8.0/21

Define the DHCP address pool as needed, any valid IPs begin and end ranges within the subnet CIDR

Address Pool = 10.33.11.1 --> 10.33.12.100

Exclude none

2. Change DHCP ranges to match IPAM subnet CIDR

Configure IPAM with a subnet 10.33.8.0/21

Define the DHCP address pool "equal to" the subnet CIDR and set up excluded addresses to cover the ranges we reserve for statically-assigned devices

Address Pool = 10.33.8.1 --> 10.33.15.254

Exclude = 10.33.8.1 --> 10.33.10.255 and 10.33.12.101 --> 10.33.15.254

3. Create a supernet to a new matching expanded DHCP scope subnets

Configure IPAM with a supernet 10.33.8.0/21

Configure IPAM subnets

10.33.8.0/23

10.33.10.0/24

10.33.11.0/24

10.33.12.0/24

10.33.13.0/24

10.33.14.0/23

Define the DHCP address pool "equal to" the subnets and set up excluded addresses to cover the ranges we reserve for statically-assigned devices

Address Pool = 10.33.11.0 --> 10.33.12.255

Exclude = 10.33.11.0, 10.33.11.255, 10.33.12.0, 10.33.12.255

Waste = 10.33.10.0, 10.33.10.255, 10.33.11.0, 10.33.11.255, 10.33.12.0, 10.33.13.0, 10.33.13.255, 10.33.14.0 (no IPAM management)

4. Something else?

Option 1 doesn't provided visibility in the GUI to the DHCP scope.

Option 2 behavior is unknown, I am not sure this will work, but likely the best option

Option 3 has added complexity and wastes IP addresses that are the network address and broadcast address within a subnet in the supernet. Also will need to verify there are no static IPs assigned in the newly expanded DHCP scope that falls into the CIDR IP ranges, setup reservations, etc. for those static IPs.

Option 4 please tell me the right thing to do

Thanks.

FormerMember · Accepted Answer

After working with technical support I learned there is no "best practice". We arrived at the unofficial recommendation to use either options 1 or 2. In either case IPAM does not have the ability to report on the smaller DHCP scope IP addresses uniquely and separately from the large subnet IP addresses. The larger subnet containing statically-assigned IP addresses will be reported in its entirety and hide results for the smaller DHCP scope IP addresses.

For example, on a Windows DHCP server you can right-click on the scope and select "Statistics". Those statistics will report the address pool, number of leases, excluded addresses, % in use, etc. IPAM only reports on the entire larger subnet. Therefore IPAM can't really be used to monitor the capacity threshold for a DHCP scope unless the entire subnet is the same size as the DHCP scope. I think I will put in a feature request for this to improve IPAM. I am frankly quite surprised by this limitation.

Best Practices for IPAM subnet containing DHCP Scope not at CIDR boundary

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