Using DHCP | Microsoft Windows 2000 Server Administrators Companion

DHCP is an essential service for any TCP/IP-based network that has more than a small number of clients. DHCP allows clients to boot up and automatically receive an IP address and other TCP/IP settings such as DNS servers, WINS servers, and the default gateway.

The following sections describe how to use the DHCP server provided with Windows 2000 Server. For more information about how DHCP works, see Chapter 13.

Designing DHCP Networks

It's important to design the deployment of DHCP servers in a way that fits the network. Small, nonrouted networks can use a single DHCP server. If you have a larger network, you need to consider the subnets and routers in use, and you should also split the address space between two servers to provide fault tolerance. Use the following checklist as part of your planning:

Draw a map of the network, showing each physical and logical subnet and the routers between the subnets.
If the network uses routers to subnet the network, determine whether the routers support forwarding DHCP broadcasts (most newer routers do, although this option needs to be turned on).
Plan on sectioning the IP address range between two servers to provide fault tolerance (DHCP servers can't communicate with each other, so they can't share the same addresses). Give 80 percent of the addresses to one server, and 20 percent to the other server (this is called the 80/20 rule). If one server goes down, clients can still receive IP addresses from the other server. Alternatively, you could set up a DHCP server cluster to handle 100 percent of the addresses in the preceding example.
If the link between the subnets is fast and reliable and the routers in between can be configured to forward DHCP broadcasts, you can place the second DHCP server on a different physical subnet. Otherwise, deploy a pair of DHCP servers to each physical subnet.
If you're using WINS, you can often make the primary WINS server the secondary (20 percent) DHCP server, and the secondary WINS server the primary (80 percent) DHCP server.
Don't run a DHCP server on a domain controller if you plan on using the DHCP server to update DNS records for legacy clients. Doing so creates an additional security risk (consult the DNS help topic in the Windows 2000 Server help system for more information—specifically check the Dynamic Updates topics).
Size the servers appropriately. Microsoft recommends that a single DHCP server service no more than 10,000 clients and 1000 scopes.
Upgrade any Microsoft Windows NT 4 domain controllers to Windows 2000 Server.
Use a disk drive with fast access time, or better yet, use a RAID. DHCP servers frequently access the disk drives, and a fast storage subsystem means better DHCP performance.

Another method of protecting against DHCP server failure is to have a hot-backup. To do this, set up a DHCP server identically to the primary DHCP server, except with its own scope that encompasses 20 percent of the address space (or possibly less). Don't activate this server (it could be serving other roles that you don't want to slow down unless it's an emergency). If the primary DHCP server goes down, manually bring the backup server online by activating its scope. However, splitting the address range between two live servers is a superior solution because it provides automatic fault tolerance—no manual intervention is required.

Plan the IP Address Range and Exclusions

Besides determining how to place the DHCP servers into the network structure, you also need to plan the IP address ranges you'll use, as well as which IP addresses to reserve or exclude from this pool of addresses. Use the following list to help plan the IP address ranges to use and exclude:

Determine the range of IP addresses that the DHCP server will manage. Most likely, this will be a private address range such as 10.x.x.x or 192.168.x.x.
Make a list of any IP addresses to exclude to support hosts with static IP addresses. Only DHCP servers and hosts that don't work as DHCP clients need static IP addresses, although you might want to use static addresses for DNS servers as well.
Consider switching all hosts with statically assigned IP addresses to DHCP assigned addresses. If any servers need unchanging addresses, create client reservations for the servers using DHCP (lease reservations are covered later in this chapter). This allows the servers to use an unchanging address and yet still have all TCP/IP options configured automatically through DHCP. It also makes it easier to track and manage IP addresses, and it will make your life much easier if your company ever needs to change its addressing, as might happen after a merger. The fewer static addresses, the better.
Make a list of servers that need to have unchanging IP addresses, such as DNS and WINS servers.
If the DHCP server will be using Internet-registered IP addresses (you probably won't), register the IP addresses with the Internet service provider (ISP).

Installing the DHCP Service

To install the DHCP service on a Windows 2000 Server computer, follow these steps:

Use the Network and Dial-Up Connections folder to manually configure a static IP address on any network interfaces for which the DHCP server will provide DHCP services (changing network settings is discussed in Chapter 6).
Open the Add/Remove Programs Control Panel tool, and then click Add/Remove Windows Components to launch the Windows Components Wizard.
Select Networking Services from the list of components, and then click Details.
Select the check box next to the Dynamic Host Configuration Protocol (DHCP) component, as shown in Figure 14-1, and then click OK. Click Next to install the service. All the necessary files are copied to the hard disk. Click Finish to close the Windows Components Wizard.

You can also use the Configure Your Server Wizard to install the DHCP service and start the Create New Scope Wizard.

Figure 14-1. Installing the DHCP service.

Creating a New Scope

Now you're ready to launch the DHCP Manager and create a new scope of IP addresses for the DHCP server to manage (a scope is a range of IP addresses that the DHCP server can manage). Before you do this, make sure you know which range of IP addresses is approved, which IP addresses need to be excluded for systems with static IP addresses, and which IP addresses need to be reserved for DNS or WINS servers.

To open DHCP Manager and create the new scope, follow these steps:

Launch DHCP from the Administrative Tools folder on the Programs menu.
If the DHCP server you want to administer isn't listed in the DHCP console tree, choose the Add Server command from the Action menu to locate the server you want to administer.
Select the appropriate DHCP server in the console tree. Select the Action menu and choose New Scope to launch the New Scope Wizard.

Real World

Scopes, Superscopes, and Multicast Scopes

A scope is simply the range of possible IP addresses on a network. To add more clients to a network where the scope is exhausted, you can add an additional scope as long as the scope doesn't belong to the same subnet as an existing scope. An excellent source of information on the complex subject of choosing subnet masks and other TCP/IP issues is Microsoft Windows 2000 TCP/IP Protocols and Services Technical Reference by Thomas Lee and Joseph Davies (Microsoft Press, 2000).

When you create multiple scopes, it's important to understand that clients from one logical subnet aren't able to obtain IP addresses from a different scope than the one they currently belong to, because the other scope is in a different logical subnet. If this is the behavior you want, great. However, if you want clients to be able to use addresses from other scopes, use a superscope.

A superscope is a collection of scopes grouped together into a single administrative whole. There are three primary reasons you might want to use superscopes:
- One scope is running out of IP addresses.
- You need to renumber the IP network, and therefore move clients from one set of addresses to another.
- You want to use two DHCP servers on the same subnet for redundancy.
When you create a superscope, you enable clients to obtain or renew leases from any scope within the superscope, even if they contain addresses from a different logical subnet.

A multicast scope is simply a scope of multicast addresses (class D addresses) that are then shared by many computers (members of the multicast group).
Click Next, and type a name and description for the scope to use for distinguishing this scope from others (Figure 14-2). Click Next.

Figure 14-2. The Scope Name screen of the New Scope Wizard.
Type the IP address that the scope begins with in the Start IP Address field, and type the IP address that the scope ends with in the End IP Address field, as shown in Figure 14-3.

Figure 14-3. The IP Address Range screen of the New Scope Wizard.
Type the network's subnet mask in the Subnet Mask box, or use the Length box to adjust the length of the subnet mask. Then click Next.
To exclude a range of addresses from the scope, in the Start IP Address box, type the start IP address for the exclusion range; in the End IP Address box, type the end IP address for the exclusion range (to add a single address, enter it in the Start IP Address box and then click Add). Then click Add. Add as many exclusions as needed, and click Next when you're finished.
Specify the lease duration for the clients, and then click Next.

Real World

Setting Lease Durations

Use longer leases for networks without redundant DHCP servers to permit more time to recover an offline DHCP server before clients lose their leases, or to minimize network traffic at the expense of less frequent address turnover. You can also use longer leases if scope addresses are plentiful (at least 20 percent available), the network is stable, and computers are rarely moved.

In contrast, scopes that support dial-up clients or mobile clients such as laptops or PDAs can have shorter leases and therefore function well with fewer addresses.

Lease durations are set in the scope's Properties dialog box. Scopes are listed in the DHCP administration console.
To configure DHCP options, choose Yes; otherwise, choose No, and then click Next. If you select No, click Finish to complete the setup of the scope.

Real World

DHCP Options

There are four different levels at which you can configure DHCP options in a Windows 2000 DHCP server:
- Server options These options apply to all clients of the DHCP server you're managing. Because of this, use server options sparingly—only specify server options for parameters common across all scopes on the server.
- Scope options These are the most often used type of options—they apply to all clients within a scope and override any server options set on the DHCP server.
- Client options These options are specified for individual clients. This is useful if specific computers need special options. Client options override all other options, including class options.
- Class options These options provide DHCP parameters to clients based on the type of client they are—either using vendor classes such as all Microsoft Windows 95/98 clients, or with user classes that specify clients with particular needs, such as RRAS clients, "mobile clients," or any other user-defined class. Class options aren't used very often. For more information about creating and using class options, consult the Help system, or the Microsoft Windows 2000 Server Resource Kit TCP/IP Core Networking Guide.
If you chose to specify DHCP options for this scope, type the routers (default gateways) you want clients to use in the Router (Default Gateway) screen, clicking Add after typing each one. When you're finished adding gateways, click Next.
Set only the options you know you need. If you're uncertain about an option, leave it alone.
Type the domain name of the domain in the Parent Domain box, and add the IP addresses for the DNS servers in the IP Address box, as shown in Figure 14-4, clicking Add after entering each one. Click Next when you're done.

Figure 14-4. The Domain Name And DNS Servers screen of the New Scope Wizard.
In the WINS Server Address box, type the addresses of any WINS servers you configured on the network for resolving NetBIOS names into IP addresses for downlevel clients. Click Next.
To activate the scope immediately, choose Yes; otherwise, choose No to activate the scope later. Click Next, and then click Finish to complete the scope configuration.

Authorizing the DHCP Server and Activating Scopes

After you set up the DHCP server and create the scopes, you must authorize the server to give leases and activate the scopes. (If installed on a domain controller, the DHCP server is authorized automatically when you add the server to the DHCP Manager console.)

Authorizing a DHCP server is an important step that Windows 2000 Server provides to reduce the occurrence of unauthorized (rogue) servers set up to hand out false IP addresses to clients. Although rogue DHCP servers that use UNIX or a hardware-based DHCP server could still be set up, Windows 2000-based DHCP servers can't be used without authorization (which also means no Windows 2000 DHCP servers in a workgroup).

To authorize the DHCP server after installing the service, follow these steps:

In the DHCP Manager, right-click DHCP at the root of the console tree and choose Manage Authorized Servers from the shortcut menu.
Click Authorize in the Manage Authorized Servers dialog box.
Type the name or IP address for the server in the text box provided and click OK.
Verify that the information is correct in the dialog box displayed, and then click Yes. Click OK to close the Manage Authorized Servers dialog box.

To activate a scope, right-click the scope in the console tree, and then choose Activate from the shortcut menu.

Don't activate a scope until you finish selecting all the options you want. Once you activate a scope, the Activate command on the menu changes to Deactivate.

Don't deactivate a scope unless you're permanently retiring the addresses from the network. If you want to temporarily disable a scope, adjust the exclusions for the scope instead. This permits existing clients to keep their addresses while preventing any further leases in the scope.

Adding Address Reservations

Reservations are handy items that you can use instead of static IP addresses (which require you to create exclusions in the DHCP scope) for all hosts that need to maintain a specific IP address, such as DNS and WINS servers (DHCP servers actually do need static IP addresses). Using reservations instead of static addresses guarantees that a server has a consistent IP address, while also providing the ability to recover the IP address in the future if the server is decommissioned or moved.

To add an address reservation to a scope, follow these steps:

Right-click the Reservations folder under the desired scope, and choose New Reservation from the shortcut menu.
Type a name for the reservation in the Reservation Name box.
Type the IP address for the client in the IP Address box, and type the MAC address for the client in the MAC Address box, as shown in Figure 14-5.
When reserving an address for a client, make the reservation on all DHCP servers that potentially service that client.

Figure 14-5. The New Reservation dialog box.
Type a description for the reservation in the Description box.
Determine which type of client you want to allow to use the reservation by selecting DHCP Only, BOOTP Only, or Both. Click Add.

To obtain the MAC address, go to the client computer (or make a remote desktop connection) and type ipconfig /all at the command prompt. The MAC address is listed as the physical address. Leave out the dashes when entering the MAC address in step 3 in the preceding procedure.

Enabling Dynamic Updates to a DNS Server for Earlier Clients

Microsoft Windows XP and Windows 2000 clients automatically update their forward lookup records with the DNS server after obtaining a new IP address from a DHCP server. No DHCP-DNS interaction is required for this behavior. However, Windows XP and Windows 2000 clients explicitly request that the DHCP server update their reverse lookup (PTR) records on the DNS server, which requires communication between the DHCP and DNS servers.

Earlier clients such as Windows Me, Windows 98, and Windows NT clients can't update their resource records themselves, so the DHCP server does it for them—if you turn on this feature (which is recommended). When this feature is enabled, the DHCP server updates an earlier client's forward and reverse lookup records when the client obtains a new IP address.

To enable earlier clients to function properly with DHCP and dynamic DNS, use the following procedure:

Select the scope or DHCP server on which you want to permit dynamic DNS updates.
From the Action menu, choose Properties, and then click the DNS tab.
Make sure that the Automatically Update DHCP Client Information In DNS check box is selected, as shown in Figure 14-6.

Figure 14-6. The DNS tab of the Properties dialog box for a DHCP scope.

This enables the DHCP server to update DNS records for clients, based on the following settings (select the Enable Updates For DNS Clients That Do Not Support Dynamic Update check box, which isn't selected by default):
- Update DNS Only If DHCP Client Requests This option allows Windows 2000 and other Windows clients to decide whether to update the DNS server themselves or have the DHCP server update their DNS address (A) and reverse lookup (PTR) records (by default, Windows 2000 and other Windows clients update their own A records, but they have the DHCP server update their PTR records).
- Always Update DNS This option forces the DHCP server to update the A and PTR records for all DHCP clients, even if they request permission to update the DNS server themselves.
- Discard Forward (Name-To-Address) Lookups When Lease Expires This option forces the DHCP server to delete the A record as well as the PTR record of a client whose lease has expired (the PTR record is always deleted at lease expiration, regardless of this setting).
- Enable Updates For DNS Clients That Do Not Support Dynamic Update This option enables the DHCP server to update the forward and reverse lookup records for clients that can't update their own forward lookup records (Windows Me and all other versions of Windows prior to Windows 2000). If you don't select this check box, the DNS records of pre-Windows 2000 clients aren't dynamically updated.

The Windows 2000 DHCP server might not be able to properly update DNS records for legacy clients if the DNS server isn't a Windows 2000 DNS server. However, the specifications are published in the RFC draft document "Interaction Between DHCP and DNS," so it's inevitable that other DNS servers will eventually work this way. Until then, use the all-Microsoft solution if dynamic DNS updates for earlier clients are important on your network.

Using Multiple DHCP Servers for Redundancy

It's vital that computers are able to obtain and keep IP addresses: without an IP address a computer can't communicate on a network or connect to the Internet. Most computers won't lose network connectivity immediately if the DHCP server goes down, but new computers, returning laptops, or little-used systems that haven't recently logged on might not be able to obtain network access until the DHCP server is working properly.

To help prevent these unhappy events, you should use redundant DHCP servers. The following sections show how to set up redundant servers using the traditional method of splitting the addresses between two servers, as well as the more complex, high-performance method of setting up a DHCP server cluster.

Splitting the Address Space Between Two Servers

To employ two DHCP servers for load balancing and redundancy, use the following procedure:

Create scopes for all valid IP addresses that the DHCP servers you're setting up will manage. This might mean creating a single scope (if you have a single subnet), or multiple scopes encompassing a number of subnets.
Set up exclusions so that the primary DHCP server handles 80 percent of the address pool and the secondary server handles the other 20 percent of the addresses. Thus, each server excludes the addresses available on the other server so that no address appears in both servers' address pools.
Create a superscope on both servers that contains all valid scopes for the physical subnet. To do so, select the appropriate DHCP server from the console tree, choose the New Superscope command from the Action menu, name the superscope, and then select the member scopes to include, as shown in Figure 14-7.

Figure 14-7. Creating a new superscope.

You can delete a superscope without affecting the member scopes by selecting the superscope and pressing Delete. However, deactivating a superscope deactivates all member scopes as well.

If you have a routed network with DHCP relay agents or routers that forward DHCP broadcasts between the physical subnets, you can use DHCP servers on other subnets as secondary servers. However, unless the DHCP server has at least one scope with available addresses from the client's own subnet, the client isn't able to obtain or renew an IP address lease. To make sure this doesn't happen, create two superscopes on each server, one for each logical subnet. Thus, each server owns 80 percent of the address pool for its local sub-net, and 20 percent of the address pool for the other DHCP server's local subnet.

Setting Up a DHCP Server Cluster

Although splitting the address space between two DHCP servers is an adequate way to provide redundancy and load balancing, an even more powerful solution is to set up the DHCP service to run on a Windows 2000 Server cluster. The members of the cluster equally share the DHCP service workload, and if one of the servers fails, the other servers continue to provide addresses to clients as if nothing had happened. Instead of an address space split between servers, each server in the cluster has access to the complete address space.

Using the Windows 2000 Cluster service is discussed in detail in Chapter 16, but the basic steps to get a DHCP server up and running in a cluster are as follows:

Install DHCP (as described earlier in this chapter) and set up the server cluster (as described in Chapter 16).
Microsoft recommends that you postpone creating DHCP scopes until the DHCP service is set up with the cluster, as described in the following steps.
Launch Cluster Administrator from the Administrative Tools folder on the Programs menu.
Select the cluster you want to use to host the DHCP service.
Choose the Configure Application command from the File menu to open the Configure Application Wizard.
Click Next in the first page and then choose the Use An Existing Virtual Server option and select the appropriate group. If you haven't already created a virtual server, choose the Create A New Virtual Server option and use the wizard to create a new virtual server (as discussed in Chapter 16).
In the Create Application Cluster Resource page, choose the Yes, Create A Cluster Resource For My Application Now option, choose the DHCP resource type, and then click Next.
Type a name and optionally a description for the DHCP resource and then click Next.
Click Advanced Properties, click the Dependencies tab, and then click Modify.
Double-click the IP address, physical disk, and network name that you want to use for the DHCP server cluster. These resources then appear as dependencies. Click OK when finished, optionally modify the properties of the resource, and then click OK.
Click Next in the Application Resource Name And Description page, specify where the DHCP database should be stored (most likely the physical disk you specified in step 9), and then click Next. If you select a different disk, make sure that it belongs to the group and that a dependency has been created for it.
Review the settings and click Finish to complete the wizard.
Verify that the resource is displayed in the correct group, and then right-click the resource and choose Bring Online from the shortcut menu to enable the DHCP server to begin servicing clients.
Authorize the DHCP server in Active Directory (if on an Active Directory network), as discussed earlier in this chapter.

Right-click the group to which the DHCP resource belongs and choose Move Group from the shortcut menu to verify that the resource has been properly created on the cluster. If the group moves properly, the resource setup is fine.

Other DHCP Functions

The DHCP snap-in, shown in Figure 14-8, provides a single point from which to administer all the properties and functionality of the DHCP servers.

Adding a DHCP Server

To add a DHCP server to the list of managed servers, right-click DHCP in the console tree and then choose Add Server from the shortcut menu. Select the server you want to add to the console and click OK.

Figure 14-8. The DHCP snap-in.

Modifying Scopes

Modify a scope's properties by right-clicking the scope in the console tree and choosing Properties from the shortcut menu. This displays the Scope Properties dialog box shown in Figure 14-9.

Figure 14-9. The General tab of the Properties dialog box for a DHCP scope.

Deactivate a scope by right-clicking the scope and choosing Deactivate from the shortcut menu. To return the scope to use, right-click the scope again and choose Activate.

To temporarily stop distribution of leases from a scope, adjust the exclusion range so that no new addresses are available rather than deactivating the scope. This avoids forcing clients currently using addresses in the scope to prematurely obtain a new IP address from a different DHCP server or a different scope.

Exclude a range of IP addresses from a scope by right-clicking the Address Pool folder under the appropriate scope and choosing New Exclusion Range from the shortcut menu. Type the range of addresses you want to exclude and then click Add.

Enabling Server-Based Conflict Detection

To enable server-based conflict detection so that the DHCP server checks an IP address before leasing it, verifying that the address isn't already in use, right-click the DHCP server in the console tree and choose Properties from the shortcut menu. Click the Advanced tab, and then set the Conflict Detection Attempts number to 1. Don't increase this number beyond 2; this results in additional searches and needlessly increases the time it takes a client to obtain an IP address.

Windows XP and Windows 2000 clients automatically verify that the IP address offered by the DHCP server is available before accepting it, so conflict detection is useful only for earlier clients. In the interest of conserving network bandwidth and reducing client startup times, leave this option disabled unless you're having problems with duplicate IP addresses, as might be the case if you have undocumented static IP addresses on the network.

Setting Up a DHCP Relay Agent

If you have a routed network, deploy DHCP servers on both sides of routers to maximize reliability and minimize bandwidth usage. However, there are instances in which you want to allow DHCP to work across a router; for example, you might want to place a secondary DHCP server on the other side of a router. Or, if the connection is very reliable and fast, as might be the case with a partitioned local network or municipal area network (MAN), you might want to simply allow clients to cross the router to reach a DHCP server.

You can configure most routers manufactured in the last several years to pass DHCP broadcasts (they're BOOTP-compliant), but if the router doesn't support forwarding DHCP broadcasts, you can set up a server running Windows 2000 Server as a DHCP relay agent using the following procedure:

Launch Routing and Remote Access from the Administrative Tools folder on the Programs menu.
If the server you want to make a DHCP relay agent isn't listed in the console tree, choose the Add Server command from the Action menu to locate the server you want to administer.
If the appropriate server isn't already enabled, select it, choose the Configure And Enable Routing And Remote Access command from the Action menu, and then click Next (otherwise skip to step 5).
Choose Manually Configured Server in the wizard, click Next, and then click Finish.
Open the IP Routing object on the desired server, right-click the DHCP Relay Agent object, and then choose New Interface from the shortcut menu.
Select the network adapter connected to the network with the wayward DHCP clients, and then click OK. Click OK in the next screen.
Right-click the DHCP Relay object again and choose Properties from the shortcut menu.
Type the address of a DHCP server you want to forward DHCP requests to and then click Add, as shown in Figure 14-10.

Figure 14-10. Setting up a DHCP relay agent.

To make a Windows NT 4 system into a DHCP relay agent, open the Network tool in Control Panel, click the Protocols tab, double-click the TCP/IP protocol item, click the DHCP Relay tab, click Add, and then type the IP address for the DHCP server on another subnet. The Windows NT system must be a member of the subnet containing the clients you want to forward to a DHCP server on another subnet and can't actually be a router itself.

Moving DHCP to Another Server

It occasionally happens that you need to move the DHCP service from one computer to another, perhaps because of a hardware upgrade (or failure) on the existing server. To prevent an interruption in the dispersing of leases, be sure there is a secondary DHCP server available during the transition.

To move a DHCP server database from one computer to another, use the following procedure:

If the DHCP console is open, close it.
Launch Services from the Administrative Tools folder on the Programs menu.
Select the DHCP Server service and click the Stop Service toolbar button, as shown in Figure 14-11.

Figure 14-11. Stopping the DHCP Server service.
Right-click the DHCP Server service and choose Properties from the shortcut menu.
Choose Disabled from the Startup list. This prevents the service from restarting when you open the DHCP console.
Copy the contents of the \%SystemRoot%\System32\Dhcp folder to a temporary folder on a network share that is accessible from the destination computer.
Install the DHCP service on the destination computer and restart the computer, if you haven't already.
Launch Services on the destination computer from the Administrative Tools folder on the Programs menu.
Select the DHCP Server service and click the Stop Service toolbar button.
Delete the contents of the \%SystemRoot%\System32\Dhcp folder on the destination computer.
Copy the Dhcp.mdb file from the temporary folder (in the System32\Dhcp folder) to the destination computer's \%SystemRoot%\System32\Dhcp folder.
Select the DHCP Server service in the destination computer's Services window and click Start Service.
Launch DHCP on the destination server from the Administrative Tools folder on the Programs menu.
Select the destination server in the DHCP console, and then choose the Reconcile All Scopes command from the Action menu.
Click Verify. Click Reconcile to reconcile leases as necessary.
Select the server and choose the Authorize command from the Action menu to authorize the server in Active Directory.

Using Ipconfig to Release, Renew, or Verify a Lease

On any Windows computer without a static IP address, you can run a command-line utility to release, renew, or verify the client's address lease. At a command prompt (or in the Run window), use one of the following commands:

To release a client's lease, type ipconfig /release.
To renew a lease, type ipconfig /renew.
To verify the client's lease, type ipconfig /all.

The Ipconfig program is useful when troubleshooting problems because it displays every detail of the current TCP/IP configuration. You can find more troubleshooting information in Chapter 38.