Some of the main topics in this chapter are
If you find the task of statically assigning IP addresses to every workstation in your environment to be challenging, impossible, or even just annoying, you should take advantage of Dynamic Host Configuration Protocol, or DHCP.
The DHCP Server service is an option for NT Server. Once installed and configured, the DHCP server can dynamically lease IP addresses to client machines on your network. Additional information, such as IP addresses of WINS servers, can be included in the DHCP information sent to the client.
DHCP provides central administration of IP addresses and other related TCP/IP information needed by the client machine. Advantages of using DHCP include:
There are four phases to IP assignment with DHCP: The client makes a request for an IP address; a DHCP server offers an IP address; the client selects an offer and requests an IP lease; and the DHCP server selected assigns an IP to the client.
2. IP lease offer(s) After requesting a lease, the DHCP client waits for a response and is said to be in a SELECTING state. Any available DHCP servers with IP addresses to offer respond to the client request with a DHCPOFFER message.
3. IP lease selection The client chooses the first DHCPOFFER it receives, regardless of which subnet the DHCP server is located on. The client then sends a broadcast DHCPREQUEST message, requesting a lease.
4. IP lease acknowledgment The DHCP server that made the offer responds to the client with a DHCPACK message while any other DHCP servers that made an offer withdraw. The IP information is assigned to the client and the protocol is bound.
The request for a lease is known as the DHCPDISCOVER message. Because the client has no IP address, the address of 0.0.0.0 is used for the source address and 255.255.255.255 is the destination. The message sent by the client contains the computer name and hardware-specific (MAC) address of the client so that the DHCP server knows which client sent the message.
The following information is contained in the DHCPOFFER message and is broadcast by any DHCP server that receives the request and has valid IP addresses to offer: the client's hardware address, an IP address, subnet mask, IP address of the DHCP server, and the length of the lease.
When the client receives an offer, it broadcasts a DHCPREQUEST message to all DHCP servers. This message tells the DHCP server that made the accepted offer that its offer has been selected. The message also tells all other DHCP servers that their offers have been turned down.
The selected DHCP server now acknowledges the client with a DHCPACK message. This message contains a valid lease for the IP address and other possible information, such as WINS server IP addresses.
All DHCP clients attempt to renew their lease when 50 percent of the lease time has expired. The client sends a DHCPREQUEST message to the DHCP server from which the lease was obtained. Because the DHCP client already knows the IP address of the DHCP server, a broadcast message is avoided.
The following steps walk you through the basic procedure for installing and configuring DHCP:
You install the DHCP Server service via the Network control panel.
Caution: The machine that will become the DHCP server must have a static IP address. This machine cannot be a DHCP client.
You are reminded that all IP-bound adapters need to be assigned static IP addresses.
4. Log on with full administrative rights and open DHCP Manager, which is found under Administrative Tools on the Start menu.
5. To activate a DHCP server, a scope must be created. A scope is a logical grouping of DHCP clients. Each subnet must have a scope, and it defines the parameters for that subnet. To create a scope, choose Scope, Create. The Create Scope dialog box appears, enabling you to configure the DHCP server.
6. Configure the scope by creating an IP address pool and subnet mask, as shown in Figure 13.3. The IP address pool begins with the first valid IP address and ends with the last, encompassing all available IP addresses.
A DHCP scope is created by entering the IP address, subnet, and lease configuration information.
NOTE: Most DHCP scopes contain excluded addresses. For example, your organization might have UNIX workstations (or other non-DHCP clients) that use static IP addresses. Using those addresses in a DHCP scope would cause an address conflict, so they're therefore excluded.
9. Options that you can configure for the scope include router IP addresses, DNS servers, and WINS servers. To configure these options, first select the desired scope, and then choose DHCP Options, Scope. Add the desired options by selecting an option and clicking Add (see Figure 13.4).
Several options can be set for each DHCP scope.
11. The dialog box that appears next varies depending on the item you're configuring. In the example in Figure 13.5, type the IP address of the new router, and click Add. You might also add a description string. Click OK when finished.
Use the IP Address Array Editor box to specify additional information in a scope, in this case a router.
13. Configuring a WINS server (see Figure 13.7) is done in the same fashion as the previous examples. If NetBIOS name resolution is going to be done on the network, WINS is the preferred method. Although you could install LMHOST files on all, a WINS database is far and away the best alternative.
See "Installing and Configuring WINS," for more information on DNS and WINS functionality in Windows NT Server, p. 225
Use the IP Address Array Editor box to specify additional information in a scope, in this case a DNS server.
Fig. 13.7 Use the IP Address Array Editor box to specify additional information in a scope, in this case a WINS server.
The following steps illustrate some of the advanced features that can be configured in a DHCP scope. Many of the features listed deal with Windows Internet Naming Service (WINS) and NetBIOS over TCP/IP (NBT). (See Chapter 14, "Implementing WINS and DNS Name Resolution," for more information on WINS.)
Microsoft networking is based on the NetBIOS naming system. All devices are given a NetBIOS name, and all network communication is accomplished via this name. Historically, Microsoft networks used the NetBEUI protocol to discover the hardware address of NetBIOS names. NetBEUI is an unroutable broadcast protocol and is therefore unsuitable for TCP/IP-based networks. To compensate for this, Microsoft developed WINS and NBT. WINS runs as a service on a Windows NT Server and uses TCP/IP to encapsulate NetBIOS requests for WINS clients, while NBT is a separate interface add-on for TCP/IP that performs NetBIOS functionality. Several of the scope options can be further configured to customize the environment:
2. In this example, select the WINS/NBT Node Type, and add it to the Active Options list.
3. The node type determines how the client will try to contact the WINS
server. For
example, to configure the client to use only point-to-point NetBIOS name queries,
use the hex number associated with the P-node type (see Figure 13.8), and then click
OK to continue.
Node type determines how the client will contact the server.
5. In the dialog box shown in Figure 13.9, select DHCP Standard Options
from the Option Class drop-down list and then 046 WINS/NBT Node Type from
the Option Name drop-down list. Then set the default value to the H-node type (H-node
is default on all NT machines).
6. Another useful default setting is the domain name. From DHCP Manager, choose
DHCP Options, Global, and then select Domain Name and add it to active options.
7. Enter the string that represents your domain name. Click OK when finished.
8. Back at the DHCP Manager window, notice that the options you just configured are summarized, as shown in Figure 13.10.
9. To review the settings for a specific server, select it. The selected options and settings are shown on the right side of the window. In Figure 13.10, the Local subnet IP shows that you have configured a router, DNS server, WINS servers, WINS node type, and domain name.
New to Windows NT Server 4.0 is the DHCP Relay Agent, which enables Windows NT Server to relay DHCP broadcast traffic. The DHCP protocol is based on the BOOTP protocol (used mainly in UNIX environments), and therefore uses the User Datagram Protocol (UDP). Because DHCP uses this protocol, packets will not travel over a router unless the router is specially configured to enable the traffic through. These specially configured routers are generally known as BOOTP forwarding routers, and comply with RFC 1542. (See Chapter 12, "Configuring TCP/IP" for more details.)
NOTE: User Datagram Protocol (UDP) is a connectionless, unreliable transport. Don't confuse unreliable with problematic--unreliable in this context simply means the delivery of packets is not verified by the sender.
The DHCP Relay Agent is needed if the routers in your organization do not support BOOTP forwarding. When the DHCP Relay Agent is installed on a Windows NT Server, it captures DHCP and BOOTP broadcast traffic and passes it over a router to any known DHCP/BOOTP servers. The following steps illustrate how to install and configure DHCP Relay Agent:
2. Click Add, and select DHCP Relay Agent.
3. Select the Protocols tab, and select TCP/IP properties.
4. On the TCP/IP Properties sheet, select the DHCP Relay tab, and add the IP of your DHCP server or servers (see Figure 13.11).
The DHCP Relay Agent can be configured via the TCP/IP Properties dialog box from the Network control panel.
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