WINDOWS 1002000 PROFESSIONAL RESOURCE KIT [Electronic resources] نسخه متنی

Configure IP Address Assignment

This section provides an overview of the methods available for assigning IP addresses to individual Windows 2000 Professional-based clients in a TCP/IP network. It presents an explanation of each configuration method, including issues you might want to consider when implementing each method.

Choose an IP address assignment method. Windows 2000 Professional provides three methods for assignment of IP addresses to TCP/IP clients: Dynamic Host Configuration Protocol (DHCP), which automatically configures clients in an enterprise with a DHCP server; Automatic Private IP Addressing (APIPA), which automatically assigns an IP address to clients in a single-subnet environment; and manual configuration of IP addresses. Choose the method that meets your environment and client requirements. See "Overview of IP Address Assignment" later in this chapter.

If automatic host configuration is desired, and a DHCP server is available, enable DHCP. DHCP provides automatic configuration of IP addresses and other configuration options for clients in a network with one or more DHCP servers. It is the default addressing method in Windows 2000 Professional. See "Configure DHCP" later in this chapter.

If automatic IP address assignment is desired, but no DHCP server is available, enable DHCP to use Automatic Private IP Addressing (APIPA). APIPA provides automatic IP address assignment for computers on networks without a DHCP server. A Windows 2000 Professional-based client assigns itself an IP address from a reserved class B network (169.254.0.0 with the subnet mask of 255.255.0.0), which cannot directly communicate with hosts outside this subnet, including Internet hosts. This option is most suitable for small, single-subnet networks, such as a home or small office. APIPA is configured by default if no DHCP servers are available on the network. See "Configure Automatic Private IP Addressing" in this chapter.

If DHCP or APIPA cannot be used, configure IP address manually. If your network does not include a DHCP server, and APIPA cannot be used, use manual IP addressing. You must configure the IP address and subnet mask to meet your client's connectivity requirements. See "Configure IP Address Manually" in this chapter.

Overview of IP Address Assignment

Each computer on a TCP/IP network must be identified by a unique 32-bit IP address to be able to communicate on a private network or the Internet. IP addresses can be grouped into two classes of IP addresses, public IP addresses and private IP addresses. These address classes are assigned by the Internet Assigned Numbers Authority (IANA), which is responsible for the management and assignment of IP addresses on the Internet and to private organizations.

Public IP addresses

On the Internet, the IANA assigns groups of IP addresses to organizations. The organizations can then assign IP addresses within those groups to individual computers. This prevents multiple computers from having the same IP address. For a computer to be visible on the Internet, it must be reachable through a public IP address.

The public IP address for your Windows 2000 Professional-based computer can be assigned through a Dynamic Host Configuration Protocol (DHCP) server available in your enterprise network, configured manually, or provided by an Internet service provider (ISP) through a dial-up connection.

Private IP addresses

The IANA has reserved a certain number of IP addresses that are never used on the global Internet. These private IP addresses are used for networks that do not want to directly connect to the Internet, but require IP connectivity. For example, a user wanting to connect multiple Windows 2000 Professional-based computers in a home network can use the Automatic Private IP Addressing (APIPA) feature to allow each computer to automatically assign itself a private IP address. The user does not need to configure an IP address for each computer, nor is a DHCP server needed.

Internet connectivity can be obtained in a network by using private IP addressing through the use of a computer with proxy or network address translator (NAT) capabilities. Windows 2000 Professional includes the Internet Connection Sharing (ICS) feature that provides NAT services to clients in a private network. For more information about Internet Connection Sharing, see "Configure Internet Connection Sharing" later in this chapter.

Dynamic Host Configuration Protocol

A configured DHCP server provides a database of available IP addresses. The server can also be set up to provide configuration options for DHCP clients, including addresses of DNS and WINS servers, gateway addresses, and other information.

At startup, each DHCP client requests configuration data from the server, permitting automatic configuration of the IP address, subnet mask and other options. The IP address is assigned to each client for an amount of time determined at the server, called a lease, which can be periodically renewed. At conclusion of the lease, the client attempts to renew the lease, or the IP address is returned to the database and is made available to other DHCP clients. DHCP provides an efficient IP configuration option for larger networks, providing simplified client configuration, and reuse of IP addresses.

Automatic Private IP Addressing

Automatic Private IP Addressing (APIPA) is appropriate for simple networks that have only one subnet. With APIPA, if no DHCP server is available, the computer automatically assigns itself a private IP address. If a DHCP server later becomes available, the computer changes its IP address to one obtained from a DHCP server. Computers using APIPA addresses can communicate only with other computers using APIPA addresses, on the same subnet. They are not directly reachable from the Internet.

Static IP Addressing

With static IP addressing, you must manually configure the IP address. This method can be time-consuming and prone to error, especially on medium to large networks. It is recommended if DHCP and APIPA are not available or feasible.

Configure DHCP

In an effort to make implementing the TCP/IP protocol more manageable, Microsoft worked with other industry leaders to create an Internet standard called Dynamic Host Configuration Protocol (DHCP) for the automatic allocation of TCP/IP configuration. DHCP is not a Microsoft standard, but a public Request for Comments (RFC) 2131 that Microsoft has implemented.

Implementing a DHCP server within an enterprise allows a network administrator to establish a range of valid IP addresses to be used per subnet and a series of options providing configuration beyond the IP address such as the subnet mask, the default gateway, and DNS and WINS server addresses. An individual IP address from the range and its associated options are assigned dynamically to any DHCP client requesting an address. If DHCP is available company-wide, users can move from subnet to subnet and always have a valid IP address. DHCP also allows the establishment of a lease time that defines how long an IP address configuration is to remain valid. A Microsoft® Windows NT® Server version 3.5 or later computer running the DHCP service can act as a DHCP server.

NOTE

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For more information about installation and configuration of the DHCP service on Windows 2000 Server, see "Dynamic Host Configuration Protocol" in the TCP/IP Core Networking Guide.

DHCP Lease Process

The first time a Windows 2000 Professional-based client with DHCP enabled starts and attempts to join a network, it automatically follows an initialization process to obtain a lease from a DHCP server. Figure 22.1 shows the lease process.

Figure 22.1 The DHCP Lease Process

The Windows 2000 Professional DHCP client requests an IP address by broadcasting a message (known as a DHCPDiscover message) to the local subnet.

The client is offered an address when a DHCP server responds with a message containing an IP address and configuration information for lease to the client (DHCPOffer). If no DHCP server responds to the client request, the Windows 2000 Professional-based client can proceed in two ways:




If the Automatic Private IP Addressing (APIPA) feature has not been disabled, the client self-configures a unique IP address from the range 169.254.0.1 to 169.254.255.254. For more details, see "Configure Automatic Private IP Addressing" in this chapter.

If APIPA has been disabled, the client network initialization fails. The client continues to resend DHCPDiscover messages in the background until it receives a valid lease from a DHCP server. The client makes four attempts to obtain a lease, one every five minutes.




The client indicates acceptance of the offer by selecting the offered address and replying to the server with a DHCPRequest message.

The client is assigned the address and the DHCP server sends an acknowledgment message (DHCPAck), approving the lease. Other DHCP option information, such as default gateway and DNS server addresses might be included in the message.

After the client receives acknowledgment, it configures its TCP/IP properties using any DHCP option information in the DHCPAck message, and completes the initialization of TCP/IP.

In rare cases, a DHCP server might return a negative acknowledgment to the client. This can happen if a client requests an invalid or duplicate address. If a client receives a negative acknowledgment (DHCPNak), the client must begin the entire lease process again.

Restarting a DHCP Client

When a Windows 2000 Professional-based client that previously leased an IP address restarts, it broadcasts a DHCPRequest message, containing a request for the previously assigned IP address. If the requested IP address is available, the DHCP server responds with an acknowledgment message, and the client joins the network.

If the IP address cannot be used by the client because it is no longer valid, in use by another client, or invalid because the client has been physically moved to a different subnet, the DHCP server responds with a negative acknowledgment (DHCPNak). If this occurs, the client must restart the lease process.

DHCP Lease Renewals

To ensure that addresses are not left in an assigned state when they are no longer needed, the DHCP server places an administrator-defined time limit, known as a lease duration, on the address assignment.

Halfway through the lease period, the DHCP client requests a lease renewal, and the DHCP server extends the lease. If a computer stops using its assigned IP address (for example, if a computer is moved to another network segment or is removed), the lease expires and the address becomes available for reassignment.

Configuring the Windows 2000 Professional DHCP Client

When TCP/IP is first installed, Windows 2000 Professional automatically enables the option to obtain an IP address from a DHCP server. You can disable this option if you want to manually enter IP addresses. For more information about disabling DHCP, see "Configure IP Address Manually" late in this chapter.

The IP configuration tool (Ipconfig) allows users or administrators to examine the current IP address configuration assigned to the computer, the IP address lease time, and other useful data about the TCP/IP configuration.

Configure Automatic Private IP Addressing

The new IP address autoconfiguration feature of TCP/IP in Windows 2000, known as Automatic Private IP Addressing (APIPA), allows home users and small business users to create a functioning, single subnet TCP/IP network without having to manually configure the TCP/IP protocol or set up a DHCP server.

APIPA allows a Windows 2000 Professional DHCP client to assign itself an IP address in the following circumstances:

The client is configured to obtain a lease DHCP, but a DHCP server cannot be found, is unavailable, or is not used (for example, in a small office/home office network).

The client used DHCP to obtain a lease, but attempts to renew the lease through a DHCP server have failed.

For more information about the DHCP lease and renewal process, see the previous discussion in "Configure DHCP" earlier in this chapter.

In these circumstances, a Windows 2000 Professional DHCP client autoconfigures the TCP/IP protocol with a selected IP address from the Internet Assigned Numbers Authority (IANA)-reserved class B network 169.254.0.0 with the subnet mask 255.255.0.0. The DHCP client performs duplicate address detection to ensure that the IP address it has chosen is not already in use. If the address is in use, it selects another IP address and reselects addresses up to 10 times. After the DHCP client has selected an address that is verifiably not in use, it configures the interface with this address. The client continues to check for a DHCP server in the background every five minutes, and if a DHCP server is found, the autoconfiguration information is abandoned and the configuration offered by the DHCP server is used instead.

To determine whether Automatic Private IP Addressing is currently enabled

At the command prompt, type ipconfig /all. The resulting text identifies your IP address and other information. Check the line that reads "Autoconfiguration Enabled." If the text reads "YES" and the IP address lies in the 169.254.x.x range, Automatic Private IP Addressing is enabled.

You can disable automatic private IP addressing in one of two ways:

You can manually configure TCP/IP by following the procedure outlined in the section "Configure IP Address Manually" later in this chapter. This method also disables DHCP.

You can also disable automatic private IP addressing for a given adapter (but not DHCP) by editing the registry.

You do this by adding the registry entry IPAutoconfigurationEnabled with a value of 0x0 (REG_DWORD data type) in the following subkey:

HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesTcpipParametersInterfaces<adapter>

Use a registry editor to add this entry, then shut down and restart the computer.

This registry entry is specific to each adapter. If multiple adapters are installed, you can disable APIPA for all installed adapters by setting the value of the IPAutoconfigurationEnabled entry to 0 (REG_DWORD data type) in the following registry subkey:

HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesTcpipParameters

CAUTION

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Do not use a registry editor to edit the registry directly unless you have no alternative. The registry editors bypass the standard safeguards provided by administrative tools. These safeguards prevent you from entering conflicting settings or settings that are likely to degrade performance or damage your system. Editing the registry directly can have serious, unexpected consequences that can prevent the system from starting and require that you reinstall Windows 2000. To configure or customize Windows 2000, use the programs in Control Panel or Microsoft Management Console (MMC) whenever possible.

NOTE

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APIPA only assigns an IP address and subnet mask. APIPA does not assign a default gateway, the IP addresses of DNS server, or the IP addresses of WINS servers. Use APIPA only on a single subnet network that contains no routers. If your small office or home office network is connected to the Internet or an organization intranet, then do not use APIPA.

Configure IP Address Manually

If you cannot use DHCP or APIPA for IP address and subnet assignment, the IP address for the Windows 2000 Professional-based client must be manually configured. The required values include the following:

The IP address for each network adapter installed on the computer.

The subnet mask corresponding to each network adapter's local network.

To configure TCP/IP manually

In Control Panel, open Network and Dial-up Connections.

Right-click the local area connection you want to modify.

Select Properties.

In the General dialog box, select Internet Protocol (TCP/IP).

Click Properties.

In the General dialog box, select the Use the following IP address option.

Type the IP address, subnet mask, and default gateway in the respective boxes.

The network administrator must provide these values for individual users, based on the IP addressing plan for your site.




The value in the IP Address box identifies the IP address for the interface.

The value in the Subnet Mask box is used to identify the network ID for the selected network adapter.




Click OK to save the IP addressing information.

Click OK to save the connection properties.