The following steps can help you determine at what level to configure and assign DHCP options for clients on your network:
Add or define new, custom option types only if you have new software or applications that requires a nonstandard DHCP option.
If your network is large, be conservative and selective when assigning global options. These options apply to all clients of a DHCP server computer.
Use scope-level options for most options that clients are assigned. In most networks, the scope level is typically the preferred level for assigning options.
Use class options if you have a large network or groups of clients with diverse needs that are able to support membership in option classes (such as Windows 2000 clients).
Use reserved client options only for clients that have special requirements—for example, if your intranet has a DNS server that performs forwarding for resolving Internet DNS names not authoritatively managed on your network. In this case, you need to add the IP address of an external DNS server on your DNS server computer. You can configure your DNS server as a reserved client in DHCP and set this address as another reserved client option.
The DHCP service uses a bottom-up hierarchy in determining which option to enforce. This simplifies DHCP management and allows a flexible administration that can range from server-wide default settings to individualized client settings when needed for special circumstances.
Following are the basic rules of how options are used:
Active global options always apply unless overridden by scope, class, or reserved options.
Active scope options always apply to any computers obtaining a lease from that scope, unless overridden by class or reserved options.
Active class options always apply to any computers configured as members of that class, unless overridden by a reserved option.
Reserved options override all other possible options.
Statically configured values on a client override any DHCP options of any type or level.