How to use the dhcp scope planner
- Enter the network and prefix, such as 192.168.10.0/24. The prefix can be from /8 to /30.
- Enter how many addresses to keep for infrastructure at the bottom, for static devices right after, and as a reserve just below the broadcast address.
- Enter the number of devices that will use DHCP and the lease time in hours.
- Read the pool range and size, how many addresses are left over after your devices, and the table showing every block of the subnet.
Formula
usable addresses = 2^(32 - prefix) - 2
pool = usable - infrastructure - static devices - top reserve
pool left = pool - devices using DHCP
renewals per hour = devices / (lease hours / 2)
The network address and the broadcast address cannot be given to a host. Clients try to renew at half the lease time, so a device with an 8 hour lease asks about every 4 hours.
An office /24
A /24 holds 256 addresses and 254 are usable. Setting aside 10 for infrastructure (192.168.10.1 to .10), 20 for static devices (.11 to .30) and 5 at the top (.250 to .254) leaves a pool of 254 - 35 = 219 addresses, from 192.168.10.31 to 192.168.10.249.
With 150 devices on DHCP, 69 addresses stay free and 68.5% of the pool is in use. An 8 hour lease means each device renews every 4 hours, about 150 / 4 = 37.5 renewals an hour on average.
Why leave room
A lease stays on the books until it expires, even after a device leaves, so the number of addresses in use can run ahead of the number of devices that are on the network at one moment. Guests, phones that come and go, and test equipment all add up. The page shows the share of the pool in use so you can see how tight a plan is, but it does not decide how much room is enough.
Shorter leases free addresses sooner but cause more renewals. Longer leases are quieter but hold addresses longer. For a block with many small subnets, plan them with the VLSM Subnet Planner first, then use this page for each one.
Keeping static and dynamic apart
Putting static addresses in their own block, outside the DHCP pool, avoids a server handing out an address that a printer already uses. The layout table shows each block with its first and last address so it can be copied into a server configuration. See also the Subnet Calculator for the mask and broadcast address.
Assumptions and limits
- The layout runs from the bottom of the subnet upward: network address, infrastructure, static devices, pool, top reserve, broadcast.
- Prefixes from /8 to /30 are accepted. A /31 or /32 has no usable host range in this model.
- The figures shown are example values to replace with your own numbers, not recommendations.
- Renewals are averaged evenly over time, which real traffic is not, and rebinding and rebooting bursts are not modeled.
- DHCP options, reservations by hardware address and failover pairs are outside the page.
Frequently asked questions
What does Short by 81 mean?
You entered more DHCP devices than there are addresses in the pool, by 81. Enlarge the subnet, shorten the lease or move some devices to another scope.
Why is the first usable address not at the start of the pool?
The pool begins after the infrastructure and static blocks, so a pool starting at .31 means the first 30 usable addresses are reserved.
Is a lease of 8 hours a good choice?
It is a common starting point for offices, not a rule. Pick the lease from how often devices come and go on your network.
What goes in the reserve at the top?
Whatever you choose, such as a few addresses for future servers. The page just keeps them out of the pool.