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Topic Last Modified: 2010-07-14

All computers that are on a TCP/IP network must have an IP address on the network to work correctly. Generally, you can manually configure IP addresses at each computer, or you can install a Dynamic Host Configuration Protocol (DHCP) server that can assign IP addresses to each client computer or device on the network. No manual configuration is required on the IP phones because the phones can receive only DHCP-assigned IP addresses.

Overview of DHCP

DHCP provides an automated way to distribute and update IP addresses and other configuration information on a network. A DHCP server provides this information to a DHCP client through the exchange of a series of messages, known as the DHCP conversation or the DHCP transaction. If the DHCP server and DHCP clients are located on different subnets, a DHCP relay agent is used to facilitate the conversation.

A DHCP client is any network-enabled device that enables you to communicate with a DHCP server to obtain dynamic, leased IP configuration and related optional information. Unified Communications phones are also DHCP clients.

DHCP Search Options

A DCHP client and DHCP server are defined as follows.

  • DHCP client is network host using DHCP to obtain configuration parameters such as a network address.

  • DHCP server is a network host that returns configuration parameters to DHCP clients.

A list of DNS suffixes to be appended for use in completing unqualified DNS names used for searching and submitting DNS queries at the client for resolution. For DHCP clients, this can be set by assigning the DNS domain name option (option 15) and providing single DNS suffix for the client to append and use in searches.

In some circumstances it is preferable that a DHCP client to be configured with the domain search list Multiple DNS suffixes are supported with the use of DHCP Search Option 119.

The DHCP search option 119 is passed from the DHCP Server to the DHCP Client to specify the domain search list used when resolving hostnames with DNS. DHCP search option 119 applies only to DNS and does not apply to other name resolution mechanisms.

Table 1. DHCP Options for domain search list

DHCP Option Description


Specifies the connection-specific DNS domain suffix to be used by the DHCP client.


DNS Domain Search List option to specify the domain search list used when resolving hostnames with DNS.

The Figure 2 below illustrates how support for multiple DNS suffixes is enabled using DHCP Option 15 and DHCP search option 119.

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To enable search option 119 for Windows Server DHCP server:

  1. Open DHCP (To open DHCP, click Start, point to Settings, click Control Panel, double-click Administrative Tools, and then double-click DHCP).

  2. In the console tree, click the applicable DHCP server.

  3. On the Action menu, click Set Predefined Options.

  4. In Predefined Options and Values, click Add (Option Class Standard), and click OK.

  5. In Name, type DNS Search List.

  6. Set Code to 119 and Data Type string (it is not an array) and click OK.

  7. Right-click Scope Options, select Configure Options, check Option 119 DNS Search List.

  8. Enter a list of domain suffixes in your organization delimited by a semi-colon (contoso.com;dev.contoso.com;corp.microsoft.com).

  9. Click OK.

See Also