Microsoft Corporation Microsoft Windows Server 2003 Deployment Kit(c) Deploying Network Services 2003

Every computer on an IP network must have a unique IP address. As noted earlier, using static addressing for clients is time-consuming and prone to error. To provide an alternative for IPv4, the IETF developed the Dynamic Host Configuration Protocol (DHCP), based on the earlier bootstrap protocol (BOOTP) standard. Figure 1.9 shows the stage in the TCP/IP design process during which you decide what to use for IP configuration. Most organizations choose to use DHCP for IPv4.

Figure 1.9: Planning an IP Configuration Strategy

Although BOOTP and DHCP hosts can interoperate, DHCP is easier to configure. BOOTP requires maintenance by a network administrator, whereas DHCP requires minimal maintenance after the initial installation and configuration.

The DHCP standard, defined in RFC 2131, defines a DHCP server as any computer running the DHCP service. Compared with static addressing, DHCP simplifies IP address management because the DHCP server automatically allocates IP addresses and related TCP/IP configuration settings to DHCP-enabled clients on the network. This is especially useful on a network with frequent configuration changes — for example, in an organization that has a large number of mobile users.

The DHCP server dynamically assigns specific addresses from a manually designated range of addresses called a scope. By using scopes, you can dynamically assign addresses to clients on the network no matter where the clients are located or how often they move.

DHCP Integration with DNS and WINS

The DHCP implementation in Windows Server 2003 is closely linked to name resolution services such as the Domain Name System (DNS) service and the Windows Internet Name Service (WINS). Network administrators benefit from combining all three when planning a deployment.

If you use DHCP servers for Windows-based network clients, you must use a name resolution service. In addition to name resolution, Windows Server 2003 networks use DNS to support Active Directory. Domain-based networks supporting clients running Windows NT version 4.0 or earlier or NetBIOS applications must use WINS servers. Networks supporting a combination of clients running Windows XP, Windows 2000, Windows Server 2003, and Windows NT 4.0 must implement both WINS and DNS.

DHCP, APIPA, and IP Address Allocation

DHCP clients receive IP addresses as follows:

• Dynamic allocation — from DHCP server. After you configure DHCP, the DHCP server automatically assigns an IP address from a specified scope to a client for a finite period of time called a lease. Most clients receive a dynamic IP address.

• Static allocation — from DHCP server. For a specific computer (such as a DHCP, DNS, or WINS server, or a print server, firewall, or router), you can manually configure the TCP/IP properties, including the IP address, the DNS and WINS parameters, and default gateway information. For the static clients to be on the same subnet as other, dynamically allocated computers, the static IP addresses must be within the scope or subnet defined for dynamic address allocation. You can use the DHCP snap-in to set an exclusion range to prevent the DHCP server from dynamically allocating the static IP address.

• Client reservation — from DHCP server. By using the DHCP snap-in, you can also reserve a specific IP address for permanent use by a given DHCP client.

• Automatic allocation — APIPA. In the absence of a DHCP server, Automatic Private IP Addressing (APIPA) lets a workstation configure itself with an address in the range from 169.254.0.1 to 169.254.255.254. Computers using APIPA addresses can communicate only with other computers that are also using APIPA addresses within a single subnet. In this case, a computer has an IP address but cannot connect outside the subnet. APIPA regularly checks for the presence of a DHCP server; if it detects one, it yields to the DHCP service, which then assigns a dynamic address to replace the APIPA address. APIPA is designed primarily for simple networks with only one subnet, such as small or home-based networks. On a larger network, APIPA can be useful for identifying problems with DHCP: when a client uses an APIPA address, this indicates that a DHCP server has not been found.

• Alternate configuration — user configured. In the absence of a DHCP server, alternate configuration lets a computer use an IP address configured manually by the user. Alternate configuration is designed for a computer that is used on more than one network, such as a laptop used both at the office and at home. The user can specify an IP address on the computer's TCP/IP properties Alternate Configuration tab if at least one of the networks (for example, the home office) does not have a DHCP server and APIPA addressing is not wanted. If alternate configuration is not configured and no DHCP server is found, TCP/IP uses APIPA by default.

For more information about developing a DHCP strategy, see "Deploying DHCP" in this book.