Using DHCP to Assign IP Addresses to Devices


Topic Last Modified: 2012-08-28

All computers that are on a TCP/IP network must have an IP address on the network to work correctly. 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 allows you to communicate with a DHCP server to obtain dynamic, leased IP configuration and related, optional information. Unified communications (UC) phones are DHCP clients.

Required Ports

Make sure that the following ports are open to allow a hardware load balancer:

  • On a Registrar: 5061, 5063 (for SIP connections)

  • On Web Services: 80 (HTTP) and 443 (HTTPS)

  • On a hardware load balancer: 444 (for HTTPS between server components), and make sure that source network address translation (SNAT) is allowed through.

User Provisioning

Ensure that users have been provisioned and enabled on Lync Server. This can be done in the Lync Server Control Panel.

In Lync Server Control Panel, go to the Users tab and search for the user. Double-click the user to see if he or she is enabled and have telephony type Enterprise Voice. Both of these values need to be set for the user to be able to connect to Lync Server and make phone calls.

Dial-in Page

After the users have been provisioned, they will go to the dial-in page to set-up their PIN.

DHCP Search Options

A DHCP 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 Domain Name System (DNS) suffixes should be added to use in completing unqualified DNS names that are used to search and submit 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 a single DNS suffix for the client to append and use in searches.

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

DHCP search option 119 is passed from the DHCP server to the DHCP client to specify the domain search list used when resolving host names with DNS. DHCP search option 119 applies only to DNS; it does not apply to other name resolution mechanisms.

DHCP Options for the Domain Search List

DHCP Option Description


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


Specifies the domain search list to be used when resolving host names with DNS

To enable search option 119 for a Windows Server DHCP server, do the following:

  1. Open DHCP server: 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 <ServerRole> pool), and click OK.

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

  8. Type a list of domain suffixes in your organization separated by a semi-colons (For example,;

  9. Click OK.