Dynamic Host Configuration Protocol: Frequently Asked Questions

DHCP is an Internet Engineering Task Force (IETF) standard specified in Request for Comments (RFC) 2131 for simplifying management of host Internet Protocol (IP) configuration. The DHCP standard defines the use of DHCP servers as a way to manage dynamic allocation of IP addresses and other related configuration details for DHCP-enabled clients on your network.

DHCP provides the following benefits for administering your TCP/IP-based network:

DHCP is defined in the following IETF RFCs:

For additional standards related to DHCP, see the Dynamic Host Configuration IETF working group.

DHCP in Windows Vista, Windows XP, Windows Server 2008, and Windows Server 2003 is IETF standards compliant.

For an overview of DHCP in Windows Server 2003, see Dynamic Host Configuration Protocol for Windows Server 2003.

DHCP documentation is included with Windows Server 2003 and Windows Server 2008 (click Start, then click Help and Support). There are also DHCP sections of the Windows Server 2003 Deployment Guide and Windows Server 2003 Technical Reference.

For the product documentation resources available for DHCP in Windows Server 2008, see the Windows Server 2008 DHCP TechCenter.

For the product documentation resources available for DHCP in Windows Server 2003, see the Windows Server Dynamic Host Configuration Protocol (DHCP) TechCenter.

For a list of all the resources for DHCP in Windows Server 2008, Windows Server 2003, and Windows 2000 Server, see the Dynamic Host Configuration Protocol Web site.

Post your question in the Network Infrastructure Servers TechNet forum.

For private networks, DHCP is used to provide centralized IP address configuration information to DHCP client computers across small, medium, or enterprise networks.

For public networks, DHCP is used to provide centralized IP address configuration to DHCP client computers connecting to the Internet, such as through broadband Internet connections or through a wireless hotspot.

Current versions of Windows support the three elements of a DHCP infrastructure:

A DHCP server running Windows Server 2008 or Windows Server 2003:

Yes. BOOTP is a host configuration protocol developed before DHCP that was designed to configure diskless workstations with limited boot capabilities. The DHCP Server service in Windows Server 2008 and Windows Server 2003 supports BOOTP clients through the configuration of a BOOTP table.

The DHCP Server and DHCP Client services include the following enhancements:

For more information, see the Microsoft Windows DHCP Team Blog.

DHCPv6 is the IPv6 counterpart to DHCP for IPv6 networks. DHCPv6 can be used to assign stateful IPv6 addresses or stateless configuration settings to IPv6 hosts. DHCPv6 is defined in RFC 3315.

The DHCP Client service in Windows Vista and Windows Server 2008 supports DHCPv6. A computer running Windows Vista or Windows Server 2008 can perform both DHCPv6 stateful and stateless configuration on a native IPv6 network. The DHCP Server service supports both DHCPv6 stateless and stateful operation.

For more information, see The DHCPv6 Protocol.

DHCP is a client-server protocol in which a DHCP client requests an IP address configuration from a DHCP server on the network. DHCP servers store the set of available IP addresses and related configuration information in a database. When DHCP clients start up on a network, they must discover the DHCP servers on the network and request IP configuration information. The DHCP servers respond with an offered IP address configuration. The DHCP client then requests the use of a specific IP address configuration from among those offered, which is acknowledged by the DHCP server that offered it.

Because the DHCP server typically only allocates an IP address configuration for a specific amount of time known as the lease time, the DHCP client must renew the lease time of its IP address configuration on an ongoing basis. If the DHCP client does not renew the lease time before it expires or changes the subnet to which it is connected, it must obtain a new IP address configuration.

For a detailed technical explanation of how DHCP works, see Chapter 6 - Dynamic Host Configuration Protocol of the TCP/IP Fundamentals for Microsoft Windows online book.

All DHCP traffic uses the User Datagram Protocol (UDP). Messages from the DHCP client to the DHCP server use UDP source port 68 and UDP destination port 67. Messages from the DHCP server to the DHCP client use UDP source port 67 and UDP destination port 68.

DHCP clients and servers use the following messages to communicate during the DHCP configuration process:

DHCP servers store ranges of addresses known as scopes that correspond to the valid IP addresses that can be assigned hosts on individual subnets. A DHCP server must allocate an IP address from the correct scope to DHCP clients. When a DHCP client starts up on a network, it does not have a valid IP address that has been acknowledged by a DHCP server and therefore cannot indicate its subnet to the DHCP server. If the DHCP client is on the same subnet as the DHCP server, the DHCP server can identify the subnet from the interface on which the DHCP messages from the client were received. When the DHCP client is not on the same subnet as the DHCP server, a DHCP relay agent on the DHCP client's subnet records the IP address of the interface on which the DHCP message from the DHCP client was received before forwarding the message to a DHCP server.

For more information, see The DHCPv6 Protocol.

In Windows Server 2008 and Windows Server 2003, the DHCP Server service is integrated with Active Directory to provide authorization for DHCP servers. An unauthorized DHCP server on a network can disrupt network operations by allocating incorrect addresses or configuration options. A DHCP server that is a domain controller or a member of an Active Directory domain queries Active Directory for the list of authorized servers (identified by IP address). If its own IP address is not in the list of authorized DHCP servers, the DHCP Server service does not complete its startup sequence and automatically shuts down.

This is a common issue for network administrators who attempt to install and configure a DHCP server in an Active Directory environment without first authorizing the server.

For a DHCP server that is not a member of the Active Directory domain, the DHCP Server service sends a broadcast DHCPInform message to request information about the root Active Directory domain in which other DHCP servers are installed and configured. Other DHCP servers on the network respond with a DHCPAck message, which contains information that the querying DHCP server uses to locate the Active Directory root domain. The starting DHCP server then queries Active Directory for a list of authorized DHCP servers and starts the DHCP Server service only if its own address is in the list.

The authorization process for DHCP server computers depends on the installed role of the server on your network. There are three roles or server types for which each server computer can be installed:

If you deploy Active Directory, all computers operating as DHCP servers must be either domain controllers or domain member servers before they can be authorized and provide DHCP service to clients.

Although it is not recommended, you can use a stand-alone server as a DHCP server as long as it is not on a subnet with any authorized DHCP servers. When a stand-alone DHCP server detects an authorized server on the same subnet, it automatically stops leasing IP addresses to DHCP clients.

To authorize a DHCP server, do the following:

No. Remote access clients use the Internet Protocol Control Protocol (IPCP), defined in RFC 1332, to obtain an IP address from their remote access server. However, remote access clients running Windows Vista, Windows XP, Windows Server 2008, or Windows Server 2003 send a DHCPInform message after they have been configured with IPCP to obtain additional configuration parameters that are not included with IPCP. The DHCP Relay Agent component of Routing and Remote Access on the remote access server forwards the DHCPInform message to a DHCP server and forwards the response back to the remote access client.

The remote access server, such as a computer running either Windows Server 2008 or Windows Server 2003 and Routing and Remote Access, can use DHCP to obtain IP addresses to assign to remote access clients with IPCP.

For more information, see Using Routing and Remote Access servers with DHCP.

To configure Routing and Remote Access to use DHCP to obtain IP addresses for remote access clients, do the following:

You must configure the DHCP Relay Agent component of Routing and Remote Access with the address of at least one DHCP server. Do the following:

The DHCP Server service in Windows Server 2008 and Windows Server 2003 can perform updates on behalf of its DHCP clients to any DNS servers that support dynamic updates. This behavior can perform DNS dynamic updates for DHCP clients that do not support DNS dynamic updates. To configure DNS dynamic update behavior for a DHCP server, do the following:

Verify the settings on the DNS tab for the properties of a DHCP server. For more information, see the Windows Server 2003 Help and Support topic titled "Using DNS servers with DHCP."

The tools you use for managing DHCP are the DHCP console found in Administrative Tools and the Netsh.exe commands in the DHCP context. For more information, see DHCP tools.

Windows Server 2008 and Windows Server 2003 provides a set of DHCP server performance counters that can be used to measure and monitor various aspects of server activity, such as the following:

Use the System Monitor snap-in. For more information, see DHCP performance monitoring reference.

See the Microsoft Knowledgebase article How to move a DHCP database from a computer that is running Windows NT Server 4.0, Windows 2000, or Windows Server 2003 to a computer that is running Windows Server 2003.

For DHCP servers running Windows Server 2003, you can use the Backup and Restore commands in the DHCP console. For more information, see Backing up the DHCP database and Restoring server data.

One of the most common causes for this problem is that the DHCP server is not authorized in Active Directory. For more information, see the "DHCP Server Authorization" section of this article.

See the following topics:

The tools you use for troubleshooting DHCP servers are the following: