Configuring DHCP for the Internal Network

Anyone who has worked in a large network environment already knows and appreciates the value of DHCP (Dynamic Host Configuration Protocol) in providing dynamic IP address assignments and other network configuration settings. Although the benefits of using DHCP in a small network may not be immediately obvious to some, the technology is almost a requirement in the SBS world. This section provides an overview of DHCP and how it is used with SBS 2003.

What Is DHCP?

The DHCP protocol is defined in RFC 1541 (www.ietf.org/rfc/rfc1541.txt) and RFC 2131 (www.ietf.org/rfc/rfc2131.txt) as a way for network computers to acquire network configuration information from a remote server instead of a local configuration. In the early days of TCP/IP networking, every workstation and server was given a static IP address in the local configuration, and when the IP addressing scheme changed, each workstation had to be changed manually. In addition, a notebook computer that moved from one location to another had to have its network configuration changed each time it moved.

To automate the assigning of IP information to network clients, the BOOTP protocol was developed, which matched a machine's ethernet card MAC address to an IP address. When initializing the network interface, the workstation would send a broadcast on the network asking for a BOOTP server, and the broadcast included the MAC address of the workstation. The BOOTP server would receive the request, check against its internal database for the MAC address, and return configuration to the client so that the client could continue to connect to the network.

The BOOTP method still required a great deal of maintenance, however, in that any time a new workstation arrived, it had to be manually added to the BOOTP database. The same thing would happen if the network card in the computer was replaced.

DHCP was developed based on the BOOTP process but without many of the restrictions of BOOTP. The workstation's MAC address does not have to exist in a database on the DHCP server to get and IP address on the network, which cut down significantly on the management needed to get a network configuration working.

DHCP does more than just provide an IP address to a client workstation on the network. Many network settings can be configured on a workstation via DHCP. Table 5.1 shows a number of the settings that can be passed on to a workstation from a Microsoft DHCP server.

Table 5.1. Some Network Settings Configured Via the Microsoft DHCP Server

Setting	Description
Router	Sets the default gateway
Time server	Sets the address for a time server
DNS servers	Sets the addresses for DNS servers
Hostname	Sets the hostname for the client
DNS domain name	Sets the default DNS domain name for the client
NIS domain name	Sets the default Network Information Service domain name
NIS servers	Sets the addresses for NIS servers
NTP servers	Sets the addresses for Network Time Protocol servers
WINS/NBNS Servers	Sets the addresses for WINS and NBNS servers
SMTP servers	Sets the addresses for available Simple Mail Transport Protocol servers
POP3 servers	Sets the addresses for available Post Office Protocol servers
NNTP servers	Sets the addresses for available Network News Transport Protocols servers

DHCP can also be used to assign the same IP address to a workstation every time it makes a DHCP request. This is similar in function to the BOOTP process in that a reservation for the computer is made in the DHCP database based on the MAC address of the workstation's network card. This effectively gives the workstation a "static" IP address on the network. This is useful for network printer and other devices that need to have a fixed address on the network without having to configure the address manually on the device.

Default SBS DHCP Settings

The initial network setup for an SBS server creates a specific DHCP configuration for all installations. Table 5.2 shows the DHCP elements configured by the SBS setup and the default settings for each element.

Table 5.2. Default DHCP Settings as Configured by the SBS Setup

Content Area	Configuration	Default Settings
Scope Properties	Scope Name	SBS Scope
Scope Properties	Lease Duration	8 days
Address Pool	Address Range	192.168.x.1192.168.x.254
Address Pool	Excluded Addresses	192.168.x.1192.168.x.9
Scope Options	Router	Internal SBS IP address
Scope Options	DNS Servers	Internal SBS IP address
Scope Options	DNS Domain Name	Internal domain name
Scope Options	WINS/NBNS Servers	Internal SBS IP address
Scope Options	WINS/NBT Node Type	0x8

Keeping in line with the best practices for network configuration, the DHCP server gives a requesting client a valid IP address, points the workstation to the correct SBS IP address for DNS and WINS, and provides the correct internal domain name. The DHCP scope also excludes the first 10 addresses in the address range for machines that need a static IP address, such as the server's internal NIC.

In addition to using these settings during the initial install, these settings are updated back to these defaults when the Change IP Address Wizard is run. Running this wizard is the easiest way to rebuild a broken DHCP configuration on the server.

Best Practice: Let SBS Provide DHCP Services

When multiple devices on the network are capable of providing DHCP to the workstations on the network, the SBS server should be used to provide DHCP services. The SBS DHCP service is preconfigured with all the settings needed for workstations to connect successfully with the SBS network. Even if a third-party DHCP server can provide all the settings options (some do not support all the DHCP options), if any network configuration changes are made, the SBS server automatically updates the necessary settings in its DHCP configuration, where the third-party device would have to be manually configured.

Customizing DHCP Settings

For most environments, the default DHCP server settings preconfigured by the SBS wizards are sufficient to handle the workstation configuration needs. There may be times when specific network requirements necessitate an adjustment to these default settings. Only in those cases should the default settings for DHCP be modified at all.

DHCP settings are modified through the DHCP Management Console, which can be launched by choosing Start, Administrative Tools, DHCP or by entering dhcpmgmt.msc after choosing Start, Run or at a command prompt. In this console, you can also view the current DHCP address leases, as shown in Figure 5.4.

Figure 5.4. The DHCP Management Console shows the current DHCP address leases on the network.

Creating Additional Address Exclusions

One common modification to the default DHCP settings is to extend the range of excluded addresses. This would be done if more than 10 static IP addresses were needed in the local network. This can be achieved by either adding an additional exclusion range to the address pool or by deleting the existing exclusion range and creating a new one.

Caution

When modifying the DHCP address exclusion range, be sure to keep the IP address of the SBS internal NIC excluded.

Follow these steps to add an additional exclusion range to the DHCP address pool:

1.	Open the DHCP Management Console.
2.	Expand the server icon.
3.	Expand the Scope folder.
4.	Click on the Address Pool folder to view the existing address and exclusion ranges (see Figure 5.5). Figure 5.5. The existing address and exclusion ranges in the DHCP address pool.
5.	Right-click on Address Pool and select New Exclusion Range.
6.	Enter the starting and ending IP addresses to exclude (see Figure 5.6). Figure 5.6. Specifying the starting and ending IP addresses to exclude from the DHCP scope.
7.	Click Add; then click Close.

Any new DHCP requests processed by the server will not receive an IP address in the exclusion range that was just added. Any existing DHCP address leases that may exist in the new exclusion range will not be affected until the workstation attempts to renew the lease for that address.

Creating DHCP Address Reservations

Another common modification to the default DHCP settings is adding a DHCP address reservation for a specific device. This is done when a network device gets its configuration information from DHCP but needs to have the same IP address every time it comes on the network. To reserve an address for a specific device, you will need the MAC address from the ethernet card for the device. Follow these steps to add a new reservation to the DHCP configuration:

1.	Open the DHCP Management Console.
2.	Expand the server icon.
3.	Expand the Scope folder.
4.	Click on the Reservations folder to see any reservations that may already exist.
5.	Right-click on Reservations and select New Reservation.
6.	Provide the Reservation Name, IP Address, MAC Address, and Description in the New Reservation dialog box (see Figure 5.7). Figure 5.7. The settings for a new reservation entered in the DHCP Management Console.
7.	Select the DHCP Only radio button. If BOOTP is used on the network, select either the Both or the BOOTP Only radio button instead.
8.	Click Add; then click Close.

The next time the device requests a new DHCP address or attempts to renew its DHCP lease, it will be given the address specified in the reservation.

Finding the MAC Address

For a Windows workstation, you can get the MAC address for the network card by running ipconfig /all in a command window. If the device is on the network and you know the IP address it has received from DHCP, you can run arp a from a command prompt on the server and get the MAC address for the device. The output from the command will be similar to the following:

C:\> arp a Interface: 192.168.16.2 --- 0x10003   Internet Address      Physical Address      Type   192.168.16.20           00-03-ff-8f-79-85       dynamic