DHCP

This help contains the following topics:

·     Introduction

·     DHCP server

¡     DHCP address pool

¡     IP address allocation sequence

¡     DHCP options

¡     IP address conflict detection

·     Configure DHCP

Introduction

The Dynamic Host Configuration Protocol (DHCP) provides a framework to assign network configuration information to network devices.

DHCP uses the client-server model. A DHCP network typically contains a DHCP server and multiple DHCP clients. If DHCP clients and the DHCP server are on different subnets, the DHCP clients can obtain configuration parameters from the DHCP server through a DHCP relay agent.

DHCP server

Use a DHCP server to assign IP addresses for the following networks:

·     A large-sized network that requires centralized management.

·     A network without sufficient IP address space. If the number of hosts is larger than the number of assignable IP addresses, not all hosts can have an IP address at the same time.

·     A network with only a few hosts requiring fixed IP addresses.

A DHCP server stores the following information in an address pool: IP addresses, lease duration, network information, domain name suffix, DNS server addresses, WINS server addresses, NetBIOS node types, and DHCP option information. The DHCP server selects an IP address and configuration parameters from the address pool and allocates them to a requesting DHCP client.

Before assigning the IP address to the client, the DHCP server performs IP address conflict detection.

DHCP address pool

The following address assignment mechanisms are available:

·     Static address binding—Manually bind the MAC address or ID of a client to an IP address in a DHCP address pool. When the client requests an IP address, the DHCP server assigns the IP address in the static binding to the client.

·     Dynamic address allocation—Specify IP address ranges in a DHCP address pool. Upon receiving a DHCP request, the DHCP server dynamically selects an IP address from the matching IP address range in the address pool.

You can specify the lease duration for statically and dynamically allocated addresses.

The DHCP server observes the following principles to select an address pool for a client:

1.     If there is an address pool where an IP address is statically bound to the MAC address or ID of the client, the DHCP server selects this address pool and assigns the statically bound IP address and other configuration parameters to the client.

2.     If the above condition is not met, the DHCP server selects an address pool depending on the client location.

¡     Client on the same subnet as the server—The DHCP server compares the IP address of the receiving interface with subnets of all address pools, and selects the address pool with the longest-matching subnet.

¡     Client on a different subnet than the server—The DHCP server compares the IP address in the giaddr field of the DHCP request with subnets of all address pools, and selects the address pool with the longest-matching primary subnet.

IP address allocation sequence

The DHCP server selects an IP address for a client in the following sequence:

1.     IP address statically bound to the client's MAC address or ID.

2.     IP address that was ever assigned to the client.

3.     IP address designated by the Option 50 field in the DHCP-DISCOVER message sent by the client.

Option 50 is the Requested IP Address option. The client uses this option to specify the wanted IP address in a DHCP-DISCOVER message. The content of Option 50 is user defined.

4.     First assignable IP address found based on the dynamic allocation rule.

5.     IP address that was a conflict or passed its lease duration. If no IP address is assignable, the server does not respond.

DHCP options

DHCP uses the options field to carry information for dynamic address allocation and provide additional configuration information for clients.

Use the DHCP options feature for the following purposes:

·     Add new DHCP options.

·     Define vendor-specific option contents. For example, you can pad vendor-specific information into Option 43.

·     Configure functions that are not supported on other DHCP Web pages. For example, you can use Option 4 to specify the IP address 1.1.1.1 as the time server address for DHCP clients.

·     Extend existing DHCP options to meet user requirements. For example, a maximum of eight DNS server addresses can be configured on the Web page. If you need to configure more DNS servers, you can use the DHCP options for extension.

Table 1 lists common DHCP options.

Table 1 Common DHCP options

IP address conflict detection

Before assigning an IP address, the DHCP server pings that IP address.

·     If the server receives a response within the specified period, it selects and pings another IP address.

·     If it receives no response, the server continues to ping the IP address until the maximum number of ping packets are sent. If still no response is received, the server assigns the IP address to the requesting client.

Configure DHCP

Configure the DHCP server

To implement the DHCP server function, perform the following tasks:

·     Enable DHCP service.

·     Configure the interface to operate in DHCP server mode.

·     Configure a DHCP address pool. You can select the dynamical allocation method or the static binding for address allocation. You can configure lease duration, domain name suffix, gateway address, and other network parameters for the DHCP address pool.