DHCP Option Parameters

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DHCP servers can be configured to provide optional data that fully configures TCP/IP on a client. Some of the most common DHCP option types configured and distributed by the DHCP server during leases include default gateway, router, DNS, and WINS parameters.

Clients can be configured with:

  • Information options. You can explicitly configure these option types and any associated values provided to clients.

  • Protocol options. You can implicitly configure these option types used by the DHCP service based on server and scope property settings.

You can use DHCP Manager to configure these properties and set them for an entire scope or for a single, reserved, client scope. The LAN Manager for OS/2 client does not support DHCP or WINS.

Information Options

Table 4.6 lists the most common types of DHCP information option types that can be configured for DHCP clients. Typically, these option types can be enabled and configured for each scope that you configure on a DHCP server.

Table 4.6 Common Information Option Types

Code

Description

3

Router

6

DNS server

15

DNS domain name

44

WINS server (NetBIOS name server)

45

NetBIOS datagram distribution server (NBDD)

46

WINS/NetBIOS node type

47

NetBIOS scope ID

Clients can receive these values to set their TCP/IP configurations, during the period of the lease.

Internal Protocol Options

Table 4.7 shows internal protocol option types that DHCP clients can be configured to use when communicating with a DHCP server to obtain or renew a lease.

Table 4.7 Common Internal Protocol Option Types

Code

Description

51

Lease time

53

DHCP message type

55

Special option type used to communicate a parameter request list to the DHCP server

58

Renewal time value (T1)

59

Rebind time value (T2)

In most cases, the actual values provided to clients with these option types are taken from the DHCP service property settings on the DHCP server.

Options for Remote Access Clients

When a remote access client obtains an IP address lease from a remote access server, run Winipcfg.exe (for Windows 95) or Ipconfig.exe (for Windows 2000 or Windows NT) to display information about the lease.

When a remote access server assigns an IP address to a remote access client, either from its own static address pool or from its cached DHCP address pool, there is no effective lease time for the IP address because it is released when the client disconnects.

However, remote access clients can still receive additional TCP/IP configuration information from the remote access server: WINS server assignments and DNS server assignments can be delegated to the client when it connects. These settings are delegated directly from the remote access server's settings. If a remote access server has WINS or DNS servers as configured entries in its dial-up connection properties, these settings are passed on to remote access clients that are DHCP-enabled.

Table 4.8 lists the DHCP option types that Windows-based clients support, which are assigned to the clients through a dial-up network connection with a remote access server.

Table 4.8 DHCP Options Used by Remote Access, Windows-Based, DHCP-Enabled Clients

Option

Description

IP Address

The remote access server proactively obtains an IP address from the DHCP server and builds a cached pool of DHCP leased addresses. The remote access server then distributes these cached IP addresses to the remote access client on demand and manages each lease. This is the only information from the DHCP server that the remote access client receives.

WINS server

Values provided with the option type are taken from the remote access server dial-up connection properties if the remote access server is configured with WINS server addresses. The client acquires the list of WINS servers that are configured on the remote access server.

DNS server

Values provided with the option type are taken from the remote access server dial-up connection properties if the remote access server is configured with DNS server addresses. The client acquires the first DNS server address listed in the remote access server's DNS server search list.

Subnet Mask

The subnet mask corresponds to the default subnet mask associated with the standard address class type (Class A, B, or C) of the given IP address.

NetBIOS Scope ID

NetBIOS scope ID information is not passed to the client. If you need to modify this setting, you must change it directly on the client.

Node Type

Node Type is not taken from the DHCP lease but can change on the remote access client, depending on WINS information. If the remote access server has no locally defined WINS servers, a b-node remote access client remains a b-node client. If the remote access server has locally defined WINS servers, a b-node remote access client switches to h-node for the duration of the connection. Windows 95 clients do not automatically switch between node types if the remote access server supplies WINS addresses. In these cases, you must manually switch the node type.