How DHCP Works

Dynamic Host Configuration Protocol (DHCP) is a client/server protocol that automatically provides an Internet Protocol (IP) host with its IP address and other related configuration information such as the subnet mask and default gateway. RFCs 2131 and 2132 define DHCP as an Internet Engineering Task Force (IETF) standard based on Bootstrap Protocol (BOOTP), a protocol with which DHCP shares many implementation details. DHCP allows hosts to obtain necessary TCP/IP configuration information from a DHCP server.

Benefits of DHCP

• Reliable IP Address configuration
• Reduced Network Administration

How DHCP Works

1.DHCP DISCOVER: The Dynamic Host Configuration Protocol (DHCP) client broadcasts a DHCP discover message on the network containing its MAC address and NetBIOS destined for UDP port 68 (used by BOOTP and Dynamic Host Configuration Protocol (DHCP) servers). This first datagram is known as a DHCPDISCOVER message, which is a request to any DHCP server that receives the datagram for configuration information.

2.DHCP OFFER: Each DHCP server on the network that receives the request responds with a DHCP offer message as broadcast (UDP port 67) to the computer that issued the DHCPDISCOVER. An offered the IP address and subnet mask is also included in the message. If the DHCP client device received multiple DHCPOFFER, the DHCP client accepts the first DHCP offer that arrives.

3.DHCP REQUEST: The Dynamic Host Configuration Protocol (DHCP) client accepts an offer and broadcasts a DHCPREQUEST datagram. The DHCPREQUEST datagram contains the IP address of the server that issued the offer and the physical address of the DHCP client. DHCPREQUEST message requests the selected DHCP server to assign the DHCP client an IP address and other TCP/IP configuration settings. DHCPREQUEST message also notifies all other DHCP servers that their offers were not accepted by the DHCP client.

4.DHCP ACK: When the DHCP server from which the offer was selected receives the DHCPREQUEST datagram, it constructs a DHCPACK datagram. This datagram is known as a DHCPACK (DHCP ACKNOWLEDGEMENT). The DHCPACK includes an IP address and subnet mask for the DHCP client. It may include other TCP/IP configuration information like IP addresses for the default gateway, IP addresses for DNS servers, IP addresses for WINS servers etc.

DHCP Terms and Definitions

Term	Definition
DHCP server	A computer running the DHCP Server service that holds information about available IP addresses and related configuration information as defined by the DHCP administrator and responds to requests from DHCP clients.
DHCP client	A computer that gets its IP configuration information by using DHCP.
Scope	A range of IP addresses that are available to be leased to DHCP clients by the DHCP Server service.
Lease	The length of time for which a DHCP client can use a DHCP-assigned IP address configuration.
Reservation	A specific IP address within a scope permanently set aside for leased use by a specific DHCP client. Client reservations are made in the DHCP database using the DHCP snap-in and are based on a unique client device identifier for each reserved entry.
Exclusionrange	One or more IP addresses within a DHCP scope that are not allocated by the DHCP Server service. Exclusions ensure that the specified IP addresses will not be offered to clients by the DHCP server as part of the general address pool.
APIPA	A TCP/IP feature in Windows XP and Windows Server 2003 that automatically configures a unique IP address from the range 169.254.0.1 through 169.254.255.254 with a subnet mask of 255.255.0.0 when the TCP/IP protocol is configured for automatic addressing, the Automatic private IP address alternate configuration setting is selected, and a DHCP server is not available. The APIPA range of IP addresses is reserved by the Internet Assigned Numbers Authority (IANA) for use on a single subnet, and IP addresses within this range are not used on the Internet.
Superscope	A configuration that allows a DHCP server to provide leases from more than one scope to clients on a single physical network segment.
Multicast IP	Multicast IP addresses allow multiple clients to receive data that is sent to a single IP address, enabling point-to-multipoint communication. This type of transmission is often used for streaming media transmissions, such as video conferencing.
Multicast Scope	A range of multicast IP addresses that can be assigned to DHCP clients. A multicast scope allows dynamic allocation of multicast IP addresses for use on the network by using the MADCAP protocol, as defined in RFC 2730.
BOOTP	An older protocol with similar functionality; DHCP is based on BOOTP. BOOTP is an established protocol standard used for configuring IP hosts. BOOTP was originally designed to enable boot configuration for diskless workstations. Most DHCP servers, including those running Windows Server 2003, can be configured to respond to both BOOTP requests and DHCP requests.