Design Options for DHCP Scopes
Applies To: Windows Server 2008
The design of a DHCP scope involves the definition of the parameters and settings to establish the boundaries of a given DHCP configuration. Several key parameters must be set, including the appropriate duration of the lease period, the support for only BOOTP clients, DHCP clients, or both, and the method for supporting DNS dynamic name updates.
In addition, DHCP provides an internal framework for passing configuration information to clients. Configuration parameters and other control information are stored in options and carried within protocol messages exchanged between the DHCP server and its clients. Options can be set per scope or in other ways described in later sections.
The default lease duration is 8 days. This duration can be changed; it can even be set to unlimited (which means the lease never expires). Shorter lease times mean that addresses can be reused more frequently and any changes to the DHCP options can be updated on clients more rapidly. However, short lease times affect the clients more critically in the event of any DHCP server downtime because it is more likely that clients will need to renew their lease while the DHCP server is unavailable.
For most clients, the default DHCP lease period of 8 days is acceptable. Shorter lease periods should be used for clients that access the network remotely, such as through RRAS, VPN, or mobile clients, because their connections will typically be short and intermittent. There is no need to bind an address for several days if the session is unlikely to last more than a few hours.
Most standard DHCP options are defined in RFCs published by the Internet Engineering Task Force (IETF). The full set of standard DHCP options are described specifically in RFC 2132, "DHCP Options and BOOTP Vendor Extensions." These options are predefined for configuration and are used by any DHCP server running the Windows Server operating system. You can also use the DHCP console to define new options at each server.
Even though most DHCP servers can assign many options, DHCP clients are typically designed to request or support only a subset of the standard options set specified by the RFCs.
How options are applied
Options are applied at different levels on each managed DHCP server:
Scope options: Applied only to clients that obtain a lease within a particular scope.
Server options: Applied for all scopes defined on a DHCP server.
Class options: Applied only to clients that are identified as members of a specified user or vendor class when obtaining a lease.
Reservation options: Applied only to a specific client computer; requires a reservation to be used in an active scope.
Commonly used options
Apart from the basic TCP/IP configuration settings, such as IP address, subnet mask, and default gateway, most clients also need the DHCP server to provide other information through DHCP options. These options include the following:
Routers: A preferred list of router IP addresses on the same subnet as DHCP clients; the clients can contact these routers as needed to forward IP packets destined for remote hosts.
DNS servers: IP addresses of DNS name servers that DHCP clients can contact and use to resolve a domain host name query.
DNS domain: The domain name that DHCP clients should use when resolving unqualified names during DNS domain name resolution.
WINS node type: A preferred NetBIOS name resolution method for the DHCP client to use, such as b-node for broadcast only or h-node for a hybrid of point-to-point and broadcast methods.
WINS server: IP addresses of primary and secondary WINS servers for the DHCP client to use.
Best practice recommendation
For most scenarios, the default lease duration of 8 days is acceptable and should not be changed unless the situation demands it. For example, where VPN clients get an address from DHCP, you might set a short lease time of a few hours. Short lease periods prevent the pool from being depleted and are appropriate in this case because most client VPN sessions are short.
The following guidelines can help you determine the level at which you need to assign the options for clients on your network:
Use scope options for assigning most options that clients use. In most networks, this is a preferred level for enabling the use of DHCP options.
Add or define custom option types only if you have software or applications that require a nonstandard DHCP option.
If your DHCP server manages many scopes for a large network, be selective when assigning server options. These options apply by default to all clients of a DHCP server computer unless otherwise overridden.
Use class options if you have a mixture of DHCP clients that are able to identify a specific class on the DHCP server when obtaining a lease. For example, a limited number of DHCP client computers running Windows 2000 can be configured to receive vendor-specific options that other clients do not use.
Use reservation options for individual DHCP clients in your network that have special configuration requirements.
You can consider excluding IP addresses for any hosts (computers or other networked devices) that do not support DHCP or are not recommended to use it; configure their IP settings manually. For example, the IP addresses of routers often must be statically configured.