WINDOWS 1002000 PROFESSIONAL RESOURCE KIT [Electronic resources] نسخه متنی

Configure Multihoming

When a computer is configured with more than one IP address, it is referred to as a multihomed system. Multihoming is supported in two different ways:

Multiple network adapters or media types per physical network.

The network adapters can be for similar or dissimilar networks. For example, a host with one Ethernet and one Token Ring adapter installed, each linked to a separate network, requires IP addresses to be bound to both adapters. There are no restrictions other than hardware.

Multiple IP addresses per network adapter.

A computer can access multiple subnets that are logically separated, but bound to a single network adapter. Such a configuration might be used in an environment where a host requires access to different divisions of a corporation network that are separated by different subnets.

Windows 2000 Professional allows an unlimited number of IP address/subnet mask pairs.

Figure 22.15 shows an example of a Windows 2000 Professional computer using multihoming to connect to two subnets. These subnets can be physically separated by disparate or disconnected cabling, or logically separated through subnetting.




Figure 22.15 Multihomed Windows 2000 Professional Computer Connected to Two Separate Networks

Windows 2000 Professional places no restrictions on physically multihomed computers, so you can add as many network adapters as the computer hardware can accommodate, and assign each a separate address.

As each new network adapter is installed, Windows 2000 Plug and Play autodetects the adapter. The device drivers for each adapter are installed, and any internal resources are automatically configured. The network adapter is bound to the TCP/IP protocol. If the network adapter is not Plug and Play-compliant, the adapter software must be installed and configured manually, using the manufacturer's instructions.

For a multihomed computer that uses multiple network adapters for physical connections to the LAN, each adapter appears as a separate adapter in the Network and Dial-up Connections folder.

For a system configured to support multiple network or media types, there are no restrictions for this type of configuration other than hardware and media support. Microsoft Windows 2000 TCP/IP supports the following:

Ethernet (Ethernet II and IEEE 802.3 SNAP encapsulation).

Token Ring (IEEE 802.5 encapsulation).

Asynchronous Transfer Mode (ATM).

Fiber Distributed Data Interface (FDDI).

WAN, using circuit-switched media such as ISDN and dial-up or dedicated asynchronous lines and virtual-circuit wide-area media such as X.25 and Frame Relay.

By default, each new network adapter is configured to use DHCP to assign an IP address to it.

To manually configure IP addresses on a multihomed system

In Control Panel, double-click Network and Dial-up Connections.

Right-click the local area network connection you want to modify, and then select Properties.

Add TCP/IP configuration information for the network adapter, as described in "Configure IP Address Manually."

Windows 2000 Professional supports multihoming through multiple addresses on a single network adapter. This configuration is useful in an environment where a single physical network is logically divided into subnets.

To configure a multihomed system using a single network adapter

In Control Panel, double-click Network and Dial-up Connections.

Right-click the local area connection you want to modify, and then select Properties.

Add TCP/IP configuration information for the first IP address, as described in "Configure IP Address Manually."

Click Advanced.

Click Add to enter the IP address and subnet mask for each additional subnet.

Each IP address and subnet mask pair is stored in the registry entries IPAddress and SubnetMask, in the subkey HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesTcpip ParametersInterfaces<adapter>.

Windows 2000 Professional allows an unlimited number of IP address/subnet mask pairs.

Multihoming Considerations

If TCP/IP is configured for multiple network adapters, or for a single network adapter with multiple IP addresses, you must consider the following issues:

NetBIOS over TCP/IP (NetBT) binds to the first IP address for each network adapter only.

When a NetBIOS name registration is sent out, only one IP address is registered per adapter. This registration occurs over the IP address that is listed first in the properties of the TCP/IP protocol for the adapter.

A unique IP address and subnet mask are defined for each adapter.

For each network adapter, an instance of TCP/IP is bound to the adapter. You can choose to have IP addresses dynamically assigned by DHCP (or APIPA if a DHCP server is not present) or defined manually as static addresses.

Domain Name System (DNS) configuration settings are global.

The settings on the DNS tab in Advanced TCP/IP Properties are used for all adapters on the computer. Therefore, for a multihomed computer, you must carefully define options for DNS that are applicable for all adapters using TCP/IP. Usually, this means that if you want to use DNS for name resolution with any TCP/IP connection, make sure DNS is configured.

Windows Internet Name Service (WINS) configuration settings are defined for each adapter.

The settings on the WINS configuration tab are used only for the adapter you are configuring. For example, NetBIOS over TCP/IP (NetBT) can be enabled or disabled for each network adapter. If you enable the option Disable NetBIOS over TCP/IP for a LAN adapter, this option is enabled only for that adapter, not for other LAN adapters on the computer.

The default gateway can be different for each adapter.

While it is possible to configure a default gateway IP address for each network interface, there is only a single active default route in the IP route table. If there are multiple default routes in the IP route table (assuming a metric of 1), then the specific default route is chosen randomly when TCP/IP is initialized. This behavior can lead to confusion and loss of connectivity.

It is recommended that when you are configuring a computer to be multihomed on two disjoint networks, configure a default gateway IP on the interface that is attached to the portion of the IP internetwork that contains the most network segments. Then, either add static routes or use a routing protocol to provide connectivity to remote networks reachable through the other interfaces.

Only one default gateway is used at a time.

Although you can have a different default gateway for each adapter, Windows 2000 Professional uses only one default gateway at a time. This means that only certain hosts are reachable:

Hosts on the local subnet

Hosts that are reachable by the default gateway

As a result, in some cases you might lose network connectivity. For example, suppose your computer is first connected to the corporate TCP/IP network and you make a PPP dial-up connection to the Internet. Your computer stops using the default gateway that connects your computer to the corporate network and instead uses the default gateway that connects your computer to the Internet. Therefore, you can reach hosts on your local subnet, but you cannot reach other hosts on your network.

To discover methods to access multiple gateways, see "Configure Local IP Routing Table" later in this chapter.