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.

vSystem support information

Support of non-default vSystems for this feature depends on the device model. This feature is available on the Web interface only if it is supported.

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 perform the following configuration tasks for the address pool:

  • Select the dynamic allocation method or the static binding for address allocation.

  • Exclude an IP address range from dynamic address allocation.

  • Configure lease duration, domain name suffix, gateway address, and other network parameters.

Configure address allocation settings

1. Navigate to the Network > DHCP > DHCP Service page.

2. Enable the DHCP service.

3. To enable the DHCP server mode on an interface, click the edit icon in the DHCP service column for the interface, and then select DHCP server.

4. Navigate to the Network > DHCP > DHCP Address Pools page.

5. Choose one of the following options as needed:

   • To edit the address allocation settings of an existing address pool, select the address pool on the box above the Address Allocation tab.

   • To create an address pool and configure its address allocation settings, click Create address pool, enter an address pool name, and then click OK.

6. Click the Address Allocation tab, and then configure the following address allocation settings as needed:

   Table-2 Address allocation configuration items

   Item	Description
   Subnet for Dynamic Allocation	Specify a subnet for dynamic address allocation.
   Address Range for Dynamic Allocation	Specify an address range for dynamic address allocation. The specified address range must belong to the specified subnet.
   Excluded Address Range	Exclude an address range from dynamic address allocation.
   Static Binding List	Bind a client ID or hardware address to an IP address. To create a static IP binding, perform the following task: Click Create, and then configure the following parameters: IP address—Specify the IP address of the static binding. Mask—Specify the mask length of the IP address. Type—Specify a client type. Options include Ethernet, Token ring, and Client ID. Hardware address—Specify the client hardware address of the static binding, a string of 4 to 39 characters. The string contains only hexadecimal numbers and hyphens (-), in the format of H-H-H…. The last H can represent two or four hexadecimal numbers while the other Hs must be all four hexadecimal numbers. Client ID—Specify the client ID of the static binding, a string of 4 to 254 characters. The string can contain only hexadecimal numbers and hyphen (-), in the format of H-H-H…. The last H can represent two or four hexadecimal number while the other Hs must be all four hexadecimal numbers. Click OK. The static binding list will display the static binding after it is created successfully.

7. Click OK.

Configure address pool options

1. Navigate to the Network > DHCP > DHCP Address Pools page.

2. Click the Address Pool Options tab, and then configure the following address pool option settings as needed:

   Table-3 Address pool option configuration items

   Item	Description
   Lease duration	Specify a limited or unlimited lease duration for dynamic address allocation.
   Domain name suffix	Specify a domain name suffix for DHCP clients. With a domain name suffix assigned, you only need to input part of a domain name on a DHCP client, and the client automatically adds the domain name suffix for name resolution.
   Gateways	Specify gateways for DHCP clients. When a DHCP client attempts to access a server or host on a remote network, it must send packets destined for the remote network to a gateway. You can specify a maximum of 64 gateways for a DHCP address pool.
   DNS servers	Specify Domain Name System (DNS) servers for DHCP clients. To access hosts on the Internet through domain names, a DHCP client must contact a DNS server to resolve names.
   WINS servers	Specify WINS servers for DHCP clients. A Microsoft DHCP client using NetBIOS protocol must contact a WINS server for name resolution.
   NetBIOS node type	Specify the NetBIOS node type for DHCP clients. When DHCP clients use NetBIOS for communication, they must create host name-to-IP mappings. The method to obtain host name-to-IP mappings varies by NetBIOS node type as follows: Broadcast—A broadcast node client sends the destination name in a broadcast message. The destination returns its IP address to the client after receiving the message. Peer-to-Peer—A peer-to-peer node client sends the destination name in a unicast message to the WINS server. The WINS server returns the destination IP address. Mixed—A mixed node client broadcasts the destination name. If it does not receive any response, it unicasts the destination name to the WINS server to get the destination IP address. Hybrid—A hybrid node client unicasts the destination name to the WINS server. If it does not receive any response, it broadcasts the destination name to get the destination IP address.
   DHCP options	Customize DHCP options. To create a DHCP option, perform the following task: Click Create, and then configure the following parameters: DHCP options—Specify an option ID. Type—Specify the type of the option content. Options include Hexadecimal string, ASCII string, and IP address. Option content—Specify the content of the DHCP option. Click OK. The DHCP option list will display the DHCP option after it is created successfully.

3. Click OK.

4. To view information about allocated IP addresses, click the Assigned Addresses tab. Available information include IP address, hardware address or client ID, and lease expiration time.