Microsoft DHCP server provides a reliable and flexible alternative to manual TCP/IP configuration. Microsoft DHCP server includes a graphical administrative tool—DHCP Manager—that allows you to define DHCP client configurations, and a database for managing assignment of client IP addresses and other optional TCP/IP configuration parameters.
You can use Microsoft DHCP server to automatically and dynamically assign TCP/IP configuration parameters to computers that start on the network. TCP/IP configuration parameters that can be dynamically assigned by a Microsoft DHCP server include:
A Microsoft DHCP server database is automatically created when the Microsoft DHCP server is installed on a computer running Windows NT Server and the TCP/IP protocol. You add data to the DHCP server database by defining DHCP scopes and DHCP options when using DHCP Manager.
A DHCP scope is an administrative grouping that identifies the configuration parameters for all DHCP clients grouped together on a physical subnet. The scope must be defined before DHCP clients can use the DHCP server for dynamic TCP/IP configuration.
To create a DHCP scope, you must use DHCP Manager to enter the following required information:
Note
Each subnet can have only one scope with a single continuous range of IP addresses. To use several address ranges within a scope, create a continuous range that encompasses all of the address ranges; then manually exclude the addresses that fall between the specific address ranges. See the section "Using DHCP Manager" later in this chapter for additional information.
In addition to the required DHCP scope information, you can define individual scope options by using DHCP Manager. The following table lists additional scope options:
Table 7.3 Additional Scope Options
Option | Description |
Deactivate | You can release an IP address if a computer is removed from a network. |
Renewal | You can change the renewal period for IP-address leases. By installation default, DHCP clients begin the renewal process when 50 percent of the IP address lease time has expired. |
Reserve | You can reserve a specific IP address for a DHCP client, such as an Internet Information Server or WINS server. An IP address can also be reserved if a computer on the network is not DHCP-enabled. Also, for TCP/IP security, computers designated as network firewalls are configured with reserved IP addresses. |
Other options, known as DHCP options, can also be configured on the DHCP server by using DHCP Manager. In addition to defining the required DHCP Scope configuration parameters, you can use Microsoft DHCP server to automatically assign advanced TCP/IP configuration options, such as a Windows Internet Name Service (WINS) server and a Domain Name System (DNS) server, to DHCP clients. To do this, you select one or more DHCP options when using DHCP Manager.
Microsoft DHCP options can be selected and assigned to a selected scope or to all scopes by clicking either Scope or Global on the DHCP Options menu. When using DHCP options, keep in mind the following guidelines:
Microsoft DHCP server provides the ability to configure many standard DHCP options as defined by RFC 1541. The following table lists the standard DHCP options available when using Microsoft DHCP server.
Note
You can use Microsoft DHCP server to set any of the options described in this table. However, Windows-based and Windows NT-based DHCP clients support only the options whose code and option name are listed in bold type. (If you use Microsoft DHCP server to administer third-party DHCP clients, you can use any option listed in this table that is supported on the third-party DHCP client.)
Table 7.4 Default DHCP Options as Defined by RFC 1541
Code | Option name | Meaning | |||||
0 | Pad | Causes subsequent fields to align on word boundaries. | |||||
255 | End | Indicates end of options in the DHCP packet. | |||||
1 | Subnet mask | Specifies the subnet mask of the client subnet. This option is defined in the DHCP Manager Create Scope or Scope Properties dialog box. It cannot be set directly in the DHCP Options dialog box. | |||||
2 | Time offset | Specifies the Universal Coordinated Time (UCT) offset in seconds. | |||||
3 | Router | Specifies a list of IP addresses for routers on the client's subnet.* | |||||
4 | Time server | Specifies a list of IP addresses for time servers available to the client.* | |||||
5 | Name servers | Specifies a list of IP addresses for name servers available to the client.* | |||||
6 | DNS servers | Specifies a list of IP addresses for DNS name servers available to the client.* Multihomed computers can have only one list per computer, not one per adapter card. | |||||
7 | Log servers | Specifies a list of IP addresses for MIT_LCS User Datagram Protocol (UDP) log servers available to the client.* | |||||
8 | Cookie servers | Specifies a list of IP addresses for RFC 865 cookie servers available to the client.* | |||||
9 | LPR servers | Specifies a list of IP addresses for RFC 1179 line-printer servers available to the client.* | |||||
10 | Impress servers | Specifies a list of IP addresses for Imagen Impress servers available to the client.* | |||||
11 | Resource Location servers | Specifies a list of RFC 887 Resource Location servers available to the client.* | |||||
12 | Host name | Specifies the host name of up to 63 characters for the client. The name must start with a letter, end with a letter or digit, and have as interior characters only letters, numbers, and hyphens. The name can be qualified with the local DNS domain name. | |||||
13 | Boot file size | Specifies the size of the default boot image file for the client, in 512-octet blocks. | |||||
14 | Merit dump file | Specifies the file and directory where the client's core image is dumped if a crash occurs. | |||||
15 | Domain name | Specifies the DNS domain name that the client should use for DNS host name resolution. | |||||
16 | Swap server | Specifies the IP address of the client's swap server. | |||||
17 | Root path | Specifies the ASCII path for the client's root disk. | |||||
18 | Extensions path | Specifies a file that can be retrieved by using Trivial File Transfer Protocol (TFTP). This file contains information interpreted the same as the vendor-extension field in the BOOTP response, except that the file length is unconstrained and references to Tag 18 in the file are ignored. | |||||
19 | IP layer forwarding | Enables or disables forwarding of IP packet for this client. 1 enables forwarding; 0 disables it. | |||||
20 | Nonlocal source routing | Enables or disables forwarding of datagrams with nonlocal source routes. 1 enables forwarding; 0 disables it. | |||||
21 | Policy filter masks | Specifies policy filters that consist of a list of pairs of IP addresses and masks specifying destination/mask pairs for filtering nonlocal source routes. Any source - routed datagram whose next-hop address does not match a filter will be discarded by the client. | |||||
22 | Max DG reassembly size | Specifies the maximum size datagram that the client can reassemble. The minimum value is 576. | |||||
23 | Default time-to-live | Specifies the default time-to-live (TTL) that the client uses on outgoing datagrams. The value for the octet is a number between 1 and 255. | |||||
24 | Path MTU aging timeout | Specifies the timeout in seconds for aging Path Maximum Transmission Unit (MTU) values (discovered by the mechanism defined in RFC 1191). | |||||
25 | Path MTU plateau table | Specifies a table of MTU sizes to use when performing Path MTU Discovered as defined in RFC 1191. The table is sorted by size from smallest to largest. The minimum MTU value is 68. | |||||
26 | MTU option | Specifies the MTU discovery size for this interface. The minimum MTU value is 68. | |||||
27 | All subnets are local | Specifies whether the client assumes that all subnets of the client's internetwork use the same MTU as the local subnet where the client is connected. 1 indicates that all subnets share the same MTU; 0 indicates that the client should assume some subnets might have smaller MTUs. | |||||
28 | Broadcast address | Specifies the broadcast address used on the client's subnet. | |||||
29 | Perform mask discovery | Specifies whether the client should use Internet Control Message Protocol (ICMP) for subnet mask discovery. 1 indicates that the client should perform mask discovery; 0 indicates that the client should not. | |||||
30 | Mask supplier | Specifies whether the client should respond to subnet mask requests using ICMP. 1 indicates that the client should respond; 0 indicates that the client should not respond. | |||||
31 | Perform router discovery | Specifies whether the client should solicit routers using the router discovery method specified in RFC 1256. 1 indicates that the client should perform router discovery; 0 indicates that the client should not use it. | |||||
32 | Router solicitation address | Specifies the IP address to which the client submits router solicitation requests. | |||||
33 | Static route | Specifies a list of IP address pairs that indicate the static routes the client should install in its routing cache. Any multiple routes to the same destination are listed in descending order or priority. The routes are destination/router address pairs. (The default route of 0.0.0.0 is an illegal destination for a static route.) | |||||
34 | Trailer encapsulation | Specifies whether the client should negotiate use of trailers (RFC 983) when using the Address Resolution Protocol (ARP). 1 indicates that the client should attempt to use trailers; 0 indicates that the client should not use trailers. | |||||
35 | ARP cache timeout | Specifies the timeout in seconds for ARP cache entries. | |||||
36 | Ethernet encapsulation | Specifies whether the client should use Ethernet (as specified by RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the interface is Ethernet. 1 indicates that the client should use RFC 1042 encapsulation; 0 indicates that the client should use RFC 894 encapsulation. | |||||
37 | Default time-to-live | Specifies the default TTL that the client should use when sending TCP segments. The minimum value of the octet is 1. | |||||
38 | Keepalive interval | Specifies the interval in seconds that the client TCP should wait before sending a keepalive message on a TCP connection. A value of 0 indicates that the client should not send keepalive messages on connections unless specifically requested by an application. | |||||
39 | Keepalive garbage | Specifies whether the client should send TCP keepalive messages with an octet of garbage data for compatibility with older implementations. 1 indicates that a garbage octet should be sent; 0 indicates that it should not be sent. | |||||
40 | NIS domain name | Specifies the name of the Network Information Service (NIS) domain as an ASCII string. | |||||
41 | NIS servers | Specifies a list of IP addresses for NIS servers available to the client.* | |||||
42 | NTP servers | Specifies a list of IP addresses for Network Time Protocol (NTP) servers available to the client.1 | |||||
43 | Vendor-specific information | Binary information used by clients and servers to exchange vendor-specific information. Servers not equipped to interpret the information ignore it. Clients that don't receive the information attempt to operate without it. | |||||
44 | WINS/NBNS servers | Specifies a list of IP addresses for NetBIOS name servers (NBNS).* | |||||
45 | NetBIOS over TCP/IP NBDD | Specifies a list of IP addresses for NetBIOS datagram distribution servers (NBDD).* | |||||
46 | WINS/NBT node type | Allows configurable NetBIOS over TCP/IP (NetBT) clients to be configured as described in RFC 1001/1002, where 1 = b-node, 2 = p-node, 4 = m-node, and 8 = h-node. On multihomed computers, the node type is assigned to the entire computer, not to individual adapter cards. | |||||
47 | NetBIOS scope ID | Specifies a text string that is the NetBIOS over TCP/IP Scope ID for the client, as specified in RFC 1001/1002. On multihomed computers, the scope ID is assigned to the entire computer, not to individual adapter cards. | |||||
48 | X Window system font | Specifies a list of IP addresses for X Window font servers available to the client.1 | |||||
49 | X Window system display | Specifies a list of IP addresses for X Window System Display Manager servers available to the client.* | |||||
51 | Lease time | Specifies the time in seconds from address assignment until the client's lease on the address expires. Lease time is specified in the DHCP Manager Create Scope or Scope Properties dialog box. It cannot be set directly in the DHCP Options dialog box. | |||||
58 | Renewal (T1) time value | Specifies the time in seconds from address assignment until the client enters the renewing state. Renewal time is a function of the lease time option, which is specified in the DHCP Manager Create Scope or Scope Properties dialog box. It cannot be set directly in the DHCP Options dialog box. | |||||
59 | Rebinding (T2) time value | Specifies the time in seconds from address assignment until the client enters the rebinding state. Rebinding time is a function of the lease time option, which is specified in the DHCP Manager Create Scope or Scope Properties dialog box. It cannot be set directly in the DHCP Options dialog box. | |||||
64 | NIS + Domain Name | Network Information Service (previously known as Yellow Pages) domain name. | |||||
65 | NIS + Servers | Network Information Service (previously known as Yellow Pages) server name. | |||||
66 | Boot Server Host Name | Identifies a Trivial File Transfer Protocol (TFTP) server. | |||||
67 | Bootfile Name | Identifies the file that is to be used as the bootfile. | |||||
68 | Mobile IP Home Agents | Can be used to list (in order of preference) IP addresses that identify mobile IP home agents. | |||||
* List is specified in order of preference.
Boldface type indicates those options that Windows-based and Windows NT-based clients support
The Microsoft DHCP network packet allocates 312 bytes for DHCP options. This is more than enough space for most option configurations. With some DHCP servers and clients, you can allocate unused space in the DHCP packet to additional options. This feature, called option overlay, is not supported by Microsoft DHCP server or client. If you attempt to use more than 312 bytes, some options settings will be lost. In that case, you should delete any unused or low-priority options.
Tip
If you are using Microsoft DHCP server to configure computers that should use the services of a WINS server, be sure to use option #44, WINS Servers, and option #46, Node Type. These DHCP options automatically configure the DHCP client as an h-node computer that directly contacts WINS servers for name registration and name query instead of using broadcasts (b-node.)
You can also add custom parameters to be included with DHCP client configuration information and change values or other elements of the predefined DHCP options, as needed.