Configuring Multiple IP Addresses and Gateways

Article
02/20/2014

Archived content. No warranty is made as to technical accuracy. Content may contain URLs that were valid when originally published, but now link to sites or pages that no longer exist.

Archived content - No warranty is made as to technical accuracy. Content may contain URLs that were valid when originally published, but now link to sites or pages that no longer exist.

from Chapter 12, Windows NT Administrator's Pocket Consultant by William R. Stanek.

Windows NT computers can have multiple IP addresses—even if the computers only have a single network adapter card. Multiple IP addresses are useful in several situations:

You want a single computer to appear to be several different computers. For example, if you're installing an intranet server, you may also want the server to provide Web, FTP, and SMTP services. You can use a different IP address for each service, and you can use different IP addresses for the intranet and the FTP services.
If your network is divided into multiple logical IP networks (subnets) and the computer needs access to these subnets to route information or provide other internetworking services, you may want a single network adapter card to have multiple IP addresses. For example, the address 192.55.10.8 could be used for workstations accessing a server from the 192.55.10.0 subnet and the address 192.55.11.8 could be used for workstations accessing a server from the 192.55.11.0 subnet.

Caution: When you use a single network adapter, IP addresses must be assigned to the same network segment or segments that are part of a single logical network. If your network is divided into multiple physical networks, you must use multiple network adapters, with each network adapter being assigned an IP address in a different physical network segment.

Assigning Addresses and Gateways

Each network adapter installed on a computer can have up to five IP addresses (a sixth IP address is configurable in some Windows NT installations). These addresses can also be associated with up to five default

Figure 12-7: Configure multiple IP addresses using the Advanced IP Addressing dialog box. Each network adapter can have up to five IP addresses and five gateways.

gateways. You assign multiple IP addresses and gateways to a single network adapter card by doing the following:

Select the Protocols tab of the Network Control Panel utility, then double-click on TCP/IP Protocol in the Network Protocols list box.
In the IP Address tab, click on the Advanced button to open the dialog box shown in Figure 12-7.
Select the network adapter you want to configure using the Adapter drop-down list. Multiple selections are only available if the computer has more than one network card/adapter installed.
Choose Add in the IP Addresses area, then enter the IP address in the IP Address field and the subnet mask in the Subnet Mask field. Repeat this step for each IP address you want to add to the network adapter card.
You can enter additional default gateways as necessary. Click on the Add button in the Gateways area and then enter the gateway address in the TCP/IP Gateway Address field. Repeat this step for each gateway you want to add.

Tip Make sure that gateways are assigned in the correct order. The gateway listed at the top of the Gateways list box is always used first. Additional gateways are used only when a gateway is unavailable. If the computer can't communicate with the first gateway, Windows NT tries to use the next gateway in the list. You can change the priority of a gateway by clicking on it and then using the Up or Down button to change the gateway order in the list box.

Configuring DNS Resolution

DNS (Domain Name Service) is a host name resolution service. You use DNS to determine the IP address of a computer from its host name. This allows users to work with host names, such as www.centraldrive.com or www.microsoft.com, rather than an IP address, such as 207.250.162.104 or 207.250.162.107. DNS is the primary name service for the Internet and is also used extensively with UNIX networks. Consequently, if a computer with a static IP address accesses resources on the Internet or on UNIX networks, you should configure DNS.

Tip A DNS server must be installed on the network (or be available to the network) for DNS to function properly. Managing DNS servers is covered in Chapter 16.

The DNS Tab of the Microsoft TCP/IP Properties Dialog Box

You configure DNS using the DNS tab of the Microsoft TCP/IP Properties dialog box shown in Figure 12-8. The fields of the DNS tab are used as follows:

Host Name Enter the TCP/IP host name of the computer, which is usually the same as the computer name. If you want to use a different TCP/IP host name, enter this name instead of the computer name.

Host names can use alphanumeric characters (A to Z and 0 to 9) as well as hyphens and periods. Host names can't use other characters. A valid host name is www.
Domain This optional field allows you to enter the DNS domain name to associate with a host. DNS domain names follow the normal hierarchy for DNS and are different from Windows NT domains.

Domain names are subject to the same naming rules as host names. A valid domain name is centraldrive.com.

Figure 12-8: Configure DNS settings using the DNS tab in the Microsoft TCP/IP Properties dialog box.

Note: To determine the fully qualified domain name for a host, the host name and domain name are combined. For example, the fully qualified domain name for the host www with a domain of centraldrive.com is www.centraldrive.com.
DNS Service Search Order Allows you to specify the IP address of the DNS servers that are used for domain name resolution. Use the Add button to add a server IP address to the list. Use the Remove button to remove a server from the list. Use the Edit button to edit the currently selected entry.

You can specify up to three servers to use for DNS resolution. These servers are used in priority order. If the first server can't resolve a particular host name, DNS attempts to use the next server on the list. If this server fails to resolve the name, the next server is used, and so on. To change the position of a server in the list box, click on it and then use the Up or Down button.
Domain Suffix Search Order Allows you to enter common domain suffixes in the Domain Suffix Search Order list box. If users fail to use a fully qualified host name, domain name suffixes can be appended to the host name during domain name resolution. A domain suffix of yahoo.com would allow the host name Quote to be resolved to quote.yahoo.com. Use the Add button to add a domain suffix to the list. Use the Remove button to remove a domain suffix from the list. Use the Edit button to edit the selected entry.

You can specify up to six domain suffixes. These suffixes are used in priority order. If the first suffix doesn't resolve properly, DNS attempts to use the next suffix in the list. If this fails, the next suffix is used, and so on. To change the order of the domain suffixes, select the suffix and then use the Up or Down button to change its position.

Configuring WINS Resolution

WINS (Windows Name Services) is used to resolve NetBIOS computer names to IP addresses. You can use WINS to help computers on a network determine the address of other computers on the network. If a WINS server is installed on the network, the server can be used to resolve computer names. WINS is only supported on computers running Microsoft Windows 3.1, Microsoft Windows 95, Microsoft Windows 98, and Windows NT.

NT computers can also be configured to use the local files LMHOSTS and HOSTS to resolve computer names. LMHOSTS can be used to resolve NetBIOS computer names. HOSTS can be used to resolve TCP/IP host names. However, these files are consulted only if normal name resolution methods fail. In a properly configured network these files are rarely used. Thus, the preferred method of NetBIOS computer name resolution is WINS in conjunction with a WINS server, and the preferred method of TCP/IP host name resolution is DNS in conjunction with a DNS server.

Figure 12-9: Configure WINS resolution for NetBIOS computer names using the WINS Address tab.

You can configure WINS by completing the following steps:

Select the Protocols tab of the Network Control Panel utility, then double-click on TCP/IP Protocol in the Network Protocols list box.
Click on the WINS Address tab. This displays the window shown in Figure 12-9.
WINS resolution is configured only for the currently selected network adapter. Select the network adapter you want to configure for WINS resolution with the Adapter drop-down list box.

Note: When a computer uses DHCP , WINS resolution is automatically enabled. While settings you make in this tab may override the settings used by DHCP , blank settings do not turn off WINS resolution. Further, if the computer you're configuring is a WINS server, WINS resolution is always handled locally.
Enter the IP address of the primary and secondary WINS servers. The optional secondary server is only used if the primary server can't respond.

Tip Windows 95 requires you to enter a primary and secondary WINS server. If the network doesn't have a secondary WINS server, simply assign the IP address of the primary server to both fields.
To enable DNS for Windows name resolution, select the Enable DNS For Windows Resolution check box. DNS For Windows Resolution is used in addition to the currently defined computer name resolution methods and is really only useful when the DNS server is configured with Windows NetBIOS computer name to IP address mappings.
To enable LMHOSTS lookups, select the Enable LMHOSTS Lookup check box. If you want the computer to use an existing LMHOSTS file defined somewhere on the network, you can retrieve this file with the Import LMHOSTS button. Generally, LMHOSTS is used only when other name resolution methods fail.

Best Practice LMHOSTS files are maintained locally on a computer-by-computer basis, which can eventually make them unreliable. Rather than relying on LMHOSTS, you should ensure that your DNS and WINS servers are configured properly and are accessible to the network. This way, you can ensure centralized administration of name resolution services.
The SCOPE ID sets the scope identifier for a computer. To limit access to a computer for NetBIOS over TCP/IP, set a SCOPE ID. Then only computers with a matching SCOPE ID will be able to communicate with the computer using NetBIOS over TCP/IP. The SCOPE ID can be a keyword or a string identifier.

Tip The concept of scope is a difficult one. It's helpful to think of a scope as a somewhat limited mechanism for restricting access to computers on a network. Using a scope, you can allow computers A, B, and C to communicate using NetBIOS over TCP/IP while preventing all other computers on the network from communicating with them using this technique. NetBIOS is used for computer browsing in Network Neighborhood, WINS, and other related services.
Repeat this process for other network adapters as necessary.

Configuring DHCP Relays

DHCP relays are used to relay BOOTP and DHCP messages to DHCP servers on different networks and subnets. Without this relay mechanism, BOOTP and DHCP messages will fail to transfer across network segments. You should configure a DHCP relay anytime DHCP clients are located on a different subnet or network from the DHCP server. Any computer that has physical connections to the needed network or subnet can act as a DHCP relay.

You can configure a DHCP relay by doing the following:

Select the Protocols tab of the Network Control Panel utility, then double-click on TCP/IP Protocol in the Network Protocols list box.
Click on the DHCP Relay tab. This displays the window shown in Figure 12-10, on the following page.
DHCP relays use the Seconds Threshold to determine which messages to relay. If a message is older than the threshold, it's discarded. Set a threshold for message aging using the Seconds Threshold field.

Figure 12-10: DHCP relays are used to relay BOOTP and DHCP messages across networks and subnets.

You can set the threshold to any value between 0 and 9999 seconds, but the default value, 4 seconds, is usually adequate. Use the Seconds and Maximum Hops thresholds together to ensure that messages aren't being relayed back and forth in an endless loop.
DHCP relays also count the number of computers that a message has passed through to get to its current position. If this number is greater than the Maximum Hops threshold, the message is discarded.

You can set the maximum hops to any value between 0 and 16. The default value, 4 hops, works well on small-sized to mid-sized networks. If you have a large network with many subnets, you may want to increase this value.
Enter the IP addresses of the DHCP servers to which messages should be relayed.
Install the DHCP Relay Agent service on the computer.

Installing the DHCP Relay Agent Service

To install the DHCP Relay Agent service, follow these steps:

Access the Services tab of the Network Control Panel utility, then click on the Add button.
Choose DHCP Relay Agent in the Select Network Service dialog box and then click OK.
Now you'll need the Windows NT distribution CD-ROM. When prompted, insert the disk and then enter the path for the distribution files, such as e:\i386\. Afterward, click Continue to allow Windows NT to retrieve the necessary files.

Configuring IP Forwarding and Dynamic Routing

When multiple network adapters are installed on a computer and those adapters are configured for separate subnets, you may want the computer to forward IP requests between the network segments. In this way, the computer can act as a simple router between the network segments.

IP Forwarding

To enable IP forwarding, follow these steps:

Select the Protocols tab of the Network Control Panel utility, then double-click on TCP/IP Protocol in the Network Protocols list box.
Click on the Routing tab, then select the Enable IP Forwarding check box.
This technique only works for static routing. If the network uses dynamic routing, you should install RIP (Routing Information Protocol).

Installing RIP for Dynamic Routing

To install the RIP service, follow these steps:

Access the Services tab of the Network Control Panel utility and then click on the Add button.
Select RIP For Internet Protocol in the Select Network Service dialog box, then click OK.
Now you'll need the Windows NT distribution CD-ROM. When prompted, insert the disk and then enter the path for the distribution files, such as e:\i386\. Afterward, click Continue.
The RIP For Internet Protocol service is now installed on the computer. You must restart the computer to turn on dynamic routing with RIP.

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