As mentioned in the previous section, DHCP (Dynamic Host Configuration Protocol) can be used to dynamically allocate IP addresses to clients as needed. Including DHCP in your network design can greatly decrease the amount of time required for configuring and debugging individual clients.
DHCP is an extension of the simpler BOOTP protocol, which is generally used by diskless workstations that boot using a disk image stored across the network. DHCP’s key advantages in a network design include the following:
IP addresses are assigned automatically rather than requiring manual configuration for each client, reducing administrative costs.
IP addresses can be managed from a central location.
When not all nodes are used concurrently, DHCP can allow a group of IP addresses to serve a larger number of nodes than the number of IP addresses.
DHCP assigns IP addresses using a lease , which grants a client the address for a specific length of time. The client must request a renewal of the lease before the lease time expires; Windows clients automatically request a renewal after 50% of the lease time has elapsed.
Any Windows 2000 computer can act as a DHCP client, and Windows 2000 Server can be configured to act as DHCP server. The DHCP Client Service is automatically configured on Windows 2000 computers and runs whether you are using DHCP or not. Windows 95/98/Me and NT, Unix, and Macintosh machines can also act as DHCP clients.
DHCP is relatively simple to use, but your network design should include the number of DHCP servers, the scopes available, and other DHCP considerations, such as placement of DHCP relay agents. These are discussed in the following sections.
DHCP uses scopes, or individual ranges of IP addresses, to provide addresses to clients. Windows 2000’s DHCP server also allows the use of superscopes : two or more scopes treated as a single pool of addresses. This is useful when the addresses available are not contiguous.
Along with IP addresses, DHCP includes options, which can configure other aspects of a client with centralized administration. Commonly used options are described in Table 28-3.
Table 28-3. Common Numeric DHCP Options
|
Option Number |
Option Name |
Value Type |
Description |
|---|---|---|---|
|
003 |
Router |
Array of IP addresses |
Preferred routers (gateways) in order of preference |
|
006 |
DNS Servers |
Array of IP addresses |
Available DNS servers in order of preference |
|
044 |
WINS/NBNS Servers |
Array of IP addresses |
Available WINS servers, in order of preference |
|
046 |
WINS/NBT Node Type |
Byte |
WINS node type; see Section 28.4.4 |
|
047 |
NetBIOS Scope ID |
String |
Identifier for NetBIOS over TCP/IP |
DHCP can be integrated with other Windows 2000 network services to provide better network management. Some services designed to integrate with DHCP are described in the following sections.
Windows 2000 includes the DHCP relay agent , which acts as a router and forwards DHCP traffic between subnets. Without this service, you must have one DHCP server per subnet; with the relay agent, a single DHCP server can handle several subnets.
DHCP can be used with Remote Access Server (RAS) to dynamically assign IP addresses to remote clients. This is ideal when you have a large pool of dial-up users; your DHCP scope needs only one IP address for each port configured for remote access, rather than one per user.
DNS converts IP hostnames to IP addresses. However, when IP addresses are dynamically assigned, the DNS server cannot keep a static list of names and addresses. Fortunately, Windows 2000’s DHCP server can automatically update the DNS server each time a client is issued a new IP address.
The DHCP server only needs to be integrated with DNS to support older (non-Active Directory) clients. Windows 2000 clients using Active Directory automatically send updates the DNS server upon being issued an IP address.
To design an optimal DHCP structure for a network, you must consider the number of servers, their placement within the network, and the need for routing or forwarding services. Methods of optimizing a DHCP design are described in the following sections.
A Windows 2000 DHCP server can manage IP addressing for several thousand clients, either all within a single subnet or using DHCP relay agent to support more than one subnet. Because DHCP uses broadcasts, relay agents must be used in any situation where a router or switch does not forward broadcast or DHCP traffic.
New to Windows 2000 is the concept of superscopes, or combinations of several scopes. This feature can be used to allow multiple DHCP servers in a single subnet (or in two or more subnets with the appropriate routing or relay agents).
To configure multiple servers, you divide the addresses in the DHCP scope into smaller scopes for each server. Then configure a superscope on each server that encompasses all of the available scopes. This is known as a distributed scope .
It is sometimes more practical to have a separate DHCP server for subnets rather than using the DHCP relay agent. In particular, when a subnet is separated from the main network by a WAN link, including a separate DHCP server in the subnet prevents delays and keeps broadcast traffic separate between the subnets.
Because DHCP is an IETF standard, Windows 2000’s DHCP server can be used to provide IP addresses to other clients, such as Unix, Macintosh, and previous versions of Windows. However, some clients may not support DHCP options, such as default gateway or DNS address assignment.
Unlike previous versions of Windows NT, Windows 2000’s DHCP server includes specific support for BOOTP clients. Because these clients use the simpler BOOTP protocol, they do not release DHCP leases; the server automatically checks whether their IP addresses are still in use when the lease expires and makes the addresses available to other clients.
A DHCP server’s performance can be improved in most of the typical ways of improving Windows 2000 performance, beginning with the basics: adding memory and adding fast disk drives or RAID arrays. The DHCP server is also multithreaded and can take advantage of multiprocessor computers.
In some cases, the computer running the DHCP service has quite a bit more power than is needed to serve a small subnet. In this case, you can install multiple network adapters and create a multihomed DHCP server. This server can provide DHCP service for several subnets without the use of forwarding or routing.
If a single server is inadequate for the needs of a network, you can add additional servers for separate subnets. In a single large subnet, you can use distributed scopes and multiple servers, as described earlier in this section.
Last but not least, you can improve DHCP server performance by setting the correct lease length in the server’s parameters. If leases are too long, an IP address shortage can result; if leases are too short, clients are required to renew frequently, which increases network traffic and places a higher load on the DHCP server.