An important part of configuring your DHCP server is configuring the options that will be included along with the IP address and subnet mask when a client makes a request. Although there were many options defined in the original DHCP specification, in reality you’ll only probably use a handful of them. I have covered the most popular ones below:
003 – Router – this option specifies the default gateway address (or addresses) to be assigned to the client, in order of preference.
006 – DNS Servers – this option specifies the IP address of DNS Servers that you wish the client to use for host name resolution, again in order of preference.
015 – DNS Domain Name – this option specifies the domain name that the client should use when resolving host names using DNS.
044 – WINS / NBNS Servers – this option specifies the IP address WINS servers to be used for Netbios name resolution.
046 – WINS / NBT Node Type – this option specifies the node type, which controls in what order the client will attempt to resolve a Netbios name to an IP address. Usually this is set to option 0x8 (h-node or hybrid) when a WINS server is used.
Note that the options above fall into the category of standard options. A new type of option, called vendor specific options also exist in Windows 2000, accessible via the advanced option tab. The Microsoft Options that you should be aware of are listed below. Note that all of these are supported on Windows 2000 clients, but may not be supported for other vendors’ systems. It is possible to add additional vendor classes, much the same as adding user classes, which will be defined shortly.
001 – Microsoft Disable Netbios Option – this option allows you to use DHCP to disable Netbios functionality on Windows 2000 clients.
002 – Microsoft Release IP Address on Shutdown Option – as the name suggests, if this option is allocated, a Windows 2000 DHCP client will fully release its IP address on shutdown, regardless of the lease duration.
003 – Microsoft Default Router Metric Base – allows you to set a default router metric, a value to be assigned to default gateway addresses on the DHCP client, used for calculating the fastest or least expensive route.
The final type of option that can be defined is what is referred to as a user class option. A user class option is one that can be created and defined. For example, I could create a special user class called ‘laptop’ and define it in DHCP.
So why would I want to do this? Simple. After defining a new class, I can then provide special options to clients of that same class. For example, I might decide that all systems that have a class ID of ‘laptop’ would have the option to release their IP address on shutdown set. But how do I set the class ID on the actual client? Simple – by using the ipconfig /setclassid command on the client. The syntax of the command is shown below:
Ipconfig /setclassid * laptop
This command will set the class ID on all my client’s network adapters to ‘laptop’. As such, when I request an IP address, I will also let the DHCP server know that I should receive all options meant for the user class ‘laptop’ as well. If the command is issued without the ‘laptop’ part, it removes the class ID from the system.
Now that you are aware of the different options that can be offered to a client, if is essential that you understand the different ways these can be allocated. The levels at which options can be allocated are Server, Scope, and Client. Options configured at the Server level apply to all scopes on the server. This provides an easy way to allocate common options, such as the address of a DNS server. Options configured at the scope level only apply to that particular scope. Finally, options configured on a client reservation apply only to the client reservation itself. In the event of conflicting settings, Server options are overridden by Scope options, which are overridden by Client options.
