As

more networks are rolled out and more computers are shipped with

multiple connection methods, the need for a workstation to participate

in more than one network is becoming more common. Working with multiple

adapters in Microsoft Windows XP can be a little intimidating at first, but it gets easier once you get into it.

This blog post is also available in PDF format in a TechRepublic download.

There are three scenarios that would require you to use more than one

network interface on a machine. In the first, you’re physically

connected to two separate networks; this would obviously require

multiple network adapters (Figure A).

Figure A

A single PC connected to two networks with separate adapters

In the second, you have two separate IP networks at the office and need to be able to access both of them (Figure B).

Figure B

A single PC connected to two networks with one adapter

In the third, you have a single network adapter but are connected to multiple networks (Figure C). For example, you could have a remote small office network with a DHCP-assigned

RFC 1918 address, but you would also need to connect to your network at

the main office via a VPN connection. In any case, you’d need to set up

XP to recognize any and all of the networks that you participate in.

Figure C

A single PC connected to two networks with one adapter but two connections

The language

You can reduce possible confusion by keeping a couple of terms in

mind while reading this article. An adapter is a piece of hardware that

you install in your system or a piece of software that you install under

XP that emulates a network adapter, such as the loopback network

adapter. Connection describes an individual connection to a network.

Depending on how your network is configured, this can include multiple

addresses. I’ll explain more about this later.

My configuration

I’m using a laptop with differing types of network adapters and

connections so that I can show you a wide range of options. I have two

physical network adapters installed — an 11-Mbps wireless adapter and a

fixed 10/100 jack on the side of the laptop. I also have a modem with a

dial-up connection that I use when I’m on the road. Other than that, I

have a whole host of virtual adapters and connections that I will also

explain.

Showing them all

In Windows XP, all network connections — dial-up, LAN, VPN, or FireWire (IEEE 1394) are shown in the Networking control panel. (Figure D)

A listing of all the Windows XP network connections

You can see in Figure D that there are four categories of network

interfaces under Windows XP. Dial-up connections are just that —

connections to a dial-up ISP. I have only one, and I use it when I can’t

connect to anything faster. Next on the list are my LAN or high-speed

Internet connections. I have two adapters: an Intel 10/100-Mbps adapter

and an 11-Mbps 802.11b D-Link wireless Ethernet adapter.

Next are Network Bridge adapters, which include my FireWire and

VMware bridge network connections. Network Bridge connections work a

little differently from other connections because they’re assigned

addresses from a pool reserved by the actual bridge adapter, which is a

piece of software. Often, bridge connections are used to communicate

between the host (the Windows XP machine) and the remote end — a device

such as a digital video camera or a certain kind of VMware session.

Finally, my VPN connections are listed. I almost always have one

connection open to my work network when I’m working at home, and I keep

Outlook running over it. (I’ve blacked out the name and IP address of

the connection for security reasons.) The last connection is

TechRepublic Tunneling Test, which I was using for testing.

Two physical connections

I’ll first go over installing two separate network adapters in a

Windows XP machine because they’re the easiest to understand and

troubleshoot. This connection method corresponds to Figure A.

In this scenario, there is a physical adapter for each individual

connection on the machine. On the laptop that I’m using to write this

article, these adapters could be considered the two physical jacks — the

wireless adapter and the one on the side of the machine.

I’ve configured the Intel adapter, which is wired directly to my home

network, to use DHCP for its address and have provided a static address

for the wireless adapter to use. However, I haven’t provided the

wireless adapter with a default gateway. Providing multiple default

gateways to a Windows 2000 or XP machine can seriously confuse network

issues, because the machine won’t know which one is the real default

gateway.

To see IP addressing information, I issue the command ipconfig /all at the command line. Listing A shows the results for the two physical network adapters in my machine.

Listing A — Windows IP Configuration

Host Name . . . . . . . . . . . . : slowe-nb
Primary Dns Suffix  . . . . . . . :
Node Type . . . . . . . . . . . . : Unknown
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix  . :
Description . . . . . . . . . . . : Intel 8255x-based PCI Ethernet Adapter (10/100)
Physical Address. . . . . . . . . : 00-20-E0-69-7F-AD
Dhcp Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IP Address. . . . . . . . . . . . : 192.168.1.102
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.1
DHCP Server . . . . . . . . . . . : 192.168.1.1
DNS Servers . . . . . . . . . . . : 209.183.205.35
   209.183.192.65
Lease Obtained. . . . . . . . . . : Sunday, July 28, 2002 4:45:53 PM
Lease Expires . . . . . . . . . . : Wednesday, July 31, 2002 4:45:53 PM
Ethernet adapter Local Area Connection 5:
       Connection-specific DNS Suffix  . :
Description . . . . . . . . . . . : D-Link DWL-650 11Mbps WLAN Adapter
Physical Address. . . . . . . . . : 00-05-5D-D9-69-30
Dhcp Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . : 10.10.10.2
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . :
NetBIOS over Tcpip. . . . . . . . : Disabled

This listing shows me the IP address, network mask, gateway, and

almost all other information related to networking that I would need.

Notice that the names of the connections correspond to the names in

Figure D. I’m also told whether this is a DHCP-assigned address.

When I attempt to ping an address on either network, Windows XP will

use the appropriate interface. When you try to traverse beyond the

routers that connect the workstation to these networks, things become a

little trickier. After the router, your Windows workstation has no way

to determine what lies beyond. It only knows about what is directly

connected to it, unless you provide it with static routes or install the

RIP Listener Service. For this reason, one of your connections must

include a default gateway. This is the device that your Windows XP

workstation will consider its “next hop” on the network when you attempt

to access services that are beyond your directly connected networks.

To modify an address on an adapter, bring up a list of network

connections by selecting Start | Control Panel | Network Connections. If

you’re using Windows XP’s default Category View, browse to Start |

Control Panel | Network And Internet Connections | Network Connections.

Note: I will not be using Category View. I find it less efficient than the classic view of the Control Panel.

Next, double-click the connection you wish to work with and click the

Properties button to bring up the information related to that adapter.

This screen will look similar to the one shown in Figure E.

Figure E

Adapter information

To bring up the TCP/IP properties, click TCP/IP and then click the

Properties button. You will see a screen similar to the one shown in Figure F.

Figure F

TCP/IP properties

To change the TCP/IP address, enter the information you need and click OK.

Single NIC, multiple networks

Next on the list of complexity is connecting a workstation to two

logically separate but physically connected networks, as shown in Figure

B. This setup might occur, for example, if (1) you have separate

departments using separate address spaces, (2) certain users need to be

able to connect to services offered by both departments, and (3)

everything is connected via switches with routers only at the edge of

the network. This setup may also be done for security reasons,

particularly when sensitive information is put on the network address

space that isn’t connected to the router that goes out to the Internet.

In any case, you’d need to be able to get workstations attached to

both networks, which is actually a very easy task. First, decide which

connection will have the default gateway. For the same reasons I

mentioned earlier, using more than one gateway can be problematic. I

always use the router with the connection out to the Internet as the

default gateway because it lets me avoid adding routes to every host on

the Internet — that wouldn’t be much fun.

Second, the address for the network connected to the Internet router

must be assigned. When you assign multiple addresses to a NIC, they all

must use static addressing. For my example, I’m going to use my wireless

adapter and work with the IP addressing. I’ll then statically assign

the second address.

To accomplish this, I’ll use a single network adapter with multiple

network addresses. (You may have heard the terms “multihoming” or

“binding multiple addresses” associated with this action.) To perform

the action, bring up the TCP/IP properties for the network adapter that

you wish to work with. On my system, the screen shown in Figure G shows my current configuration, with a single address assigned to the network adapter.

Figure G

Current configuration of my wireless adapter

To add an address, click the Advanced button, which will bring up a second properties screen, as shown in Figure H.

Figure H

Advanced TCP/IP properties

You need to work with three areas. The first is the IP Addresses

section, which is where you’ll add the second IP address. You can see in

Figure H that there is already one address assigned. The second section

lists the default gateways currently defined on the machine. You can

see that a single gateway is already defined. Finally, the network

metrics section defines the order in which network information will be

used. This information can be used to alleviate problems with multiple

default gateways, but it isn’t always 100 percent reliable.

Adding a second address

To add the second address, click the Add button in the IP Addresses

section of the window. You’ll be presented with a window that asks for

the IP address and subnet mask for the new address, as shown in Figure I.

Figure I

Adding a second IP address

That’s all you need to begin accessing resources on the second network. Listing B

shows the network parameters from an ipconfig /all command for this

adapter. Notice that there are two entries for IP address now listed.

This shows that Windows XP can communicate with both the 10.10.10 and

the 10.10.11 networks over this single physical network connection.

Listing B

Ethernet adapter Local Area Connection 5:
Connection-specific DNS Suffix  . :
Description . . . . . . . . . . . : D-Link DWL-650 11Mbps WLAN Adapter
Physical Address. . . . . . . . . : 00-05-5D-D9-69-30
Dhcp Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . : 10.10.11.2
Subnet Mask . . . . . . . . . . . : 255.255.255.0
IP Address. . . . . . . . . . . . : 10.10.10.2
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.10.10.1
NetBIOS over Tcpip. . . . . . . . : Disabled

Other types of connections

Windows XP can also work with other types of connections, such as VPN

adapters, as evidenced by Figure D. In essence, these types of

connections make use of one of the actual physical connections but show

up as a separate, virtual connection, as shown in Figure C.

In the example shown in Figure D, a VPN connection was established

over the WAN Miniport (PPTP) to a specific IP address. The WAN Miniport

is a virtual port established in XP for just this purpose. The ipconfig

/all listing for this connection is visible in Listing C. The IP

addressing information in Listing C is DHCP assigned from the VPN server

in my office, as are the remaining parameters, such as DNS servers and

WINS information. Other than the fact that it is a software adapter and

uses a real adapter to do its work, the VPN adapter works like the other

adapters I have shown you.

System tray tip

Finally, I find it useful to enable icons in the system tray to get

at-a-glance information about my network connections without having to

open the Networking control panel (Figure J).

Figure J

Network information in the system tray

This information is more useful if you rename the network connections

with something more descriptive than Local Network Connection. You can

enable a system tray icon for any adapter — hardware- or software-based —

by selecting the Show Icon In Notification Area When Connected check

box on the properties page for the adapter.