Now that the IP addressing plan, network topology, and DHCP client needs have been defined, it is time to start defining the various scopes.
When defining a scope, the most important information to define is the address range of the scope. The address range will be used by the DHCP server to determine which IP address to assign to a DHCP client. The address range is defined by the subnet the scope will be servicing. For example, if the subnet is 10.64.0.0/11, the valid range of IP addresses for this scope is 10.64.0.1 through 10.95.255.254. For any statically configured network devices on that subnet, exemptions have to be created. An exemption designates an IP address not to be assigned to a DHCP client. If a static IP address was not exempted, the DHCP server may assign the IP address to a DHCP client. As a result, an IP address conflict could occur and cause connectivity problems for the two computers involved.
If the IP addressing plan calls for using dynamic address allocation for this subnet, simply assign the address range to the scope. If the IP addressing plan calls for using manual address allocation, reservations need to be created for each network device.
Lease durations determine when the DHCP server can reclaim the allocated IP address. Usually the default time period, 8 days, is more than sufficient for most scopes. Setting the lease duration too long will cause IP addresses to be shown as allocated, thus unable to be reclaimed. Setting the lease duration too short may cause excessive DHCP traffic on the network as DHCP clients attempt to renew their address leases.
One situation that does call for a shorter lease duration is when the roaming address allocation method is being used on a scope. By specifying a short lease duration, the DHCP server will be able to reclaim IP addresses that are only in use for a short period of time, such as a conference room or library.
Any options required by the DHCP clients being serviced by the scope need to be configured at this point. Options such as the Router Option (3) need to specify the IP address for the default gateway on the subnet. Other options should be specified as well, such as the IP addresses of the DNS servers that will be servicing the subnet.