Increasing the Number of IP Addresses Available for Mobility

Technical Note 2188

Last Reviewed 14-Jun-2005
Applies to:
NetMotion Mobility 5.0 and higher
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Summary

A major requirement of the Mobility server's virtual IP addresses is that they be on the same subnet as the server itself. If your server is on a subnet with limited available addresses, you may need to make some network configuration changes to increase your IP address count. This technical note describes some options for doing this.

Change the Mobility Server's Subnet

Often the easiest solution is to put the Mobility server on a subnet that allows for more IP addresses, for example with a 23-bit mask instead of a 24-bit mask.

Add Another NIC to Your Mobility Server

A Mobility server will function correctly with multiple network cards, provided the machine's route table and the network (routers, etc.) are configured properly to route traffic on this network. You don't need to configure anything within the Mobility software to enable this. Once the machine has been configured with the added NIC, you can assign VIPs on the new subnet, and Mobility clients will be able to connect to any of the server's IP addresses.

A common mistake when configuring a server with two NICs is to misconfigure the route table. Typically the server's route table should have only one default gateway, with static routes configured for traffic that you want to go out the other interface.

Enable Supernetting on your LAN

Often an easier alternative to adding NICs to your Mobility server is to assign multiple IP addresses from different subnets to the server's single NIC. This is refered to as "supernetting," because you are using the same physical ethernet wire to carry the traffic of multiple logical subnets. As with adding a second NIC, enabling supernetting doesn't require you to change anything in the Mobility software to enable this; other than assigning the VIPs, the changes are all done in the server's TCP/IP stack and on your network.

Here are the basic steps for enabling supernetting:

1. In your server network adapter's Advanced TCP/IP properties, add the server's second IP address on the IP addresses tab. The machine must be using static IP addresses to enable this functionality.

2. Configure your VIPs on any of the subnets on which the server now resides.

3. Configure your routers for the new subnet. There are two options for this:

   • Typically your router should treat this new subnet as any other. The router needs an IP address on the new subnet, and it sends out ARP requests to locate the Mobility server when it has data destined for one of the VIPs.

   • Because the VIPs all actually reside on the Mobility server, another option is to configure the router to send all traffic destined for the new subnet directly to the Mobility server's primary IP address.

     This option isn't recommended if you have more than one Mobility server because the router would have to be configured to send traffic for part of the subnet to one server and the rest of it to the other server. This interferes with Mobility's load balancing feature, in particular when static VIPs are used.

Use a NAT device

With the Mobility server behind a NAT device, you're free to create a subnet of any size you wish without taking IP addresses away from your primary LAN. This requires, however, another piece of hardware, which might make it a more expensive solution than those above.

Related Information

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