Setting Up a Virtual IP Address Scheme

Technical Note 2166

Last Reviewed 15-Feb-2008
Applies To
Mobility client and server, version 6.00 and higher
 Printer-friendly version

About Virtual IP Addresses

When a Mobility client connects to the Mobility server, the server assigns it a virtual IP address (VIP address). Each client gets one VIP address, and the addresses reside on the server, as if the server itself had multiple IP addresses.

When a host on your internal network communicates with a client it talks to the client's VIP address, not its actual, point-of-presence IP address. This allows a host to continue communicating with the client as the client moves between networks and subnets; from the host's perspective the client is stationary, because the VIP address always resides on the server. The Mobility server handles the forwarding of traffic destined for the client's VIP address to and from the client itself.

Configuring a VIP address scheme is one of the few settings all Mobility administrators must consider when first setting up the server.

About Global vs. Server Settings

You can configure most server settings globally, making them apply to all servers in a server pool, or you can customize settings for a single server. (A few settings can be set only globally, or only at the server level.) For settings that can be configured at either the global or server level, server settings override the global setting.

In the case of the virtual IP address settings discussed below, all of them can be set at either the global or server level, with the exception of Virtual Address—Pool: it is set at the server level only.

Choosing a VIP Address Scheme

VIP addresses can be assigned in one of three ways:

• From DHCP (the default global setting)

• Using a VIP address pool

• Individual static assignments

DHCP

When configured to assign VIP addresses using DHCP, the Mobility server sends out a DHCP request for an address on the server's local subnet when a client connects. It then uses the returned IP address as the VIP address for that client. This request is similar to any machine requesting an address from DHCP; the Mobility server just happens to send out a request for each Mobility client that connects.

When using DHCP there is no guarantee that the VIP address assigned to the client will remain the same each time the client connects.

To use DHCP for VIP addresses, set the following global server setting:

Virtual IP Address Pool

A VIP address pool is a range of IP addresses the Mobility server uses to make VIP address assignments. It is similar to using DHCP in that there is no guarantee that a client will get the same VIP address each time it connects. One advantage to using a pool instead of DHCP is that it doesn't require a DHCP server on the Mobility server's LAN; when a client connects, the Mobility server just picks an available VIP address from its pool, instead of sending out a DHCP request.

There are two important requirements for the VIP addresses in a pool:

• They must be on the same subnet as your Mobility server (if your server has more than one IP address, the VIP addresses can be on any of those subnets).

• The range of IP addresses in your pool must not be in use anywhere else on your network. It shouldn't be in use by another Mobility server, either: Mobility pools are configured on a server-by-server basis, so if you have more than one Mobility server, you must have a separate pool of VIP addresses for each one.

On the Server Settings page in the Mobility console, configure the following settings:

Hint: On the Mobility server, open a command prompt and type ipconfig /all to get the settings for the subnet mask, DNS, and WINS. Remember that since the VIP addresses reside on the Mobility server, they must be on the same subnet as the server and share the same subnet mask.

You don't have to set the subnet mask, DNS, or WINS when using DHCP because their settings are taken from the DHCP response.

Individual Static Assignments

Use static VIP addresses if your users or devices must have the same IP address every time they connect to the Mobility server. Static VIP addresses aren't used by themselves—you must also allow either DHCP or a small VIP address pool for clients that haven't been assigned a static VIP address, or for clients connecting for the first time.

Important: The static VIP addresses you use cannot be used anywhere else on your network. This also means that they cannot be included in a VIP address pool or DHCP scope, and they cannot be used by another Mobility server. A common misperception is that you must set up a VIP address pool, then assign static addresses from that pool; this is not the case. The VIP address pool and the static addresses must be separate ranges, otherwise you'll run into problems where your static VIP addresses are in use by clients that got them out of a pool.

To assign static VIPs:

1. Configure the Mobility server to use either DHCP or a small VIP address pool.

   If you are assigning static VIP addresses by device, remember that the device will not appear in the Mobility console until after it connects for the first time. This means you won't be able to assign it a static VIP address in advance. Using DHCP or a VIP address pool allows the first connection, which lets you then configure the device. The client device will get the static assignment the next time it connects (if you want to force a device to reconnect to pick up the new setting, go to the list of connected devices from the Client Status page, put a check by the device, then click on Reconnect).

2. On the Server Settings page in the Mobility console, configure the following settings:

   Virtual Address—Subnet Mask	(Enter the subnet mask of your VIP addresses, which should match the one the Mobility server uses.)
   Virtual Address—DNS Servers	(Enter your internal DNS servers.)
   Virtual Address—WINS Servers	(If you are using WINS, enter your internal WINS servers.)

3. On the Client Settings page in the Mobility console, select your device or user (see note below), then:

   1. Select Virtual Address—Allocation

   2. Select the Override check box

   3. Select Use Single Static Address

   4. Enter the static VIP address that you want to assign to this client or user

   5. Click Apply

Notes on Static VIP Addresses

• Like the VIP addresses in a pool, static VIP addresses must be on the same subnet as the Mobility server, and they cannot be in use by anything else on your network.

• By default, not all devices and users are shown on the Client Settings page (this keeps it free from being cluttered with global settings). If you don't see your device or user, click on Devices or Users in the bottom left corner to see a list. Check the box next to the items to which you want to assign static VIP addresses, then click Configure. You should now see the device or user on the main Client Settings page.

• You can use a Mobility Console extension to see a list of all of your static VIP address assignments by device; see technical note 2152.

• Static VIP addresses can be assigned by device or user; generally we don't recommend that you assign them by both device and user, because then it's not clear why a client gets a particular VIP address. If you do assign them by both device and user, the user assignment will take precedence over the device assignment (as is true of all client settings).

Notes on VIP Addresses with Multiple Mobility Servers

If you have a Mobility XE server pool with more than one server, load balancing and failover are turned on by default. This means that clients may connect to a different server each time, which can affect what VIP address they are receiving:

• If using DHCP, a client will get its VIP address on the same subnet as the Mobility server to which it is connecting.

• If using VIP address pools, a client's address will change from one Mobility server to the next. This is because the pools are assigned to specific servers.

• If using static VIP addresses, a device or user will get the same static address regardless of which server it is connected to, provided the servers are on the same subnet. If you must install your Mobility servers on different subnets and you want to use static VIP addresses, load balancing must be off. You may also want to turn failover support off, depending on what your needs are (some administrators would rather allow failover with the understanding that the VIP address will change, but for others it may be essential for the client to get the same VIP address every time).

Related Information