The only drawback in reaping the benefits of virtual IP addresses is the consumption of additional IP addresses. This constraint stems from the requirement that virtual IP network addresses must be different from all other real IP network addresses. Although this constraint is not particularly severe in enterprises that use private addressing (where the IP address space is potentially large), it could become limiting in organizations that do not use private addresses.
In enterprises that use fixed-length subnetting together with a dynamic routing protocol like RIP-1, each virtual IP address could consume a large number of host IP addresses. One way to circumvent this problem is to configure a virtual IP address with a host mask of all 1s (that is, FF.FF.FF.FF), thereby consuming only one host IP address. Of course, the viability of this option depends on the ability of the RIP-1 routers on the network to recognize and honor the advertised host routes.
In autonomous systems that use variable-length subnet masking (VLSM) together with routing protocols like RIP-II or OSPF, the consumption of additional IP addresses is not a major problem. You could simply configure a virtual IP address with an IP network mask as large as possible (including a host mask of all 1s), thereby limiting the number of addresses consumed by the virtual IP address space.
In any network environment, one of the first obstacles is how clients locate and connect to the services. A business continuity cluster can exacerbate this problem because services can migrate to nodes on a completely different network segment. Although there are many potential solutions to this problem, such as DNS and SLP, none of them offers the simplicity and elegance of virtual IP addresses. With virtual IP addresses, the IP address of the service can follow the service from node to node in a single cluster, as well as from node to node in separate, distinct clusters. This makes the client reconnection problem trivial; the client just waits for the new route information to be propagated to the routers on the network. No manual steps are required, such as modifying a DNS server.
To use a virtual IP address in a business continuity cluster, we recommend using a host mask. To understand why, consider the fact that each service in a clustered environment must have its own unique IP address or, a unique virtual IP address. Furthermore, consider that each virtual IP address belongs to a virtual IP network whose route is being advertised by a single node within a cluster. Because Novell Cluster Services can migrate a service and its virtual IP address from one node to another, the virtual IP network must migrate to the same node as the service. If multiple virtual IP addresses belong to a given virtual IP network, one of two events must occur:
All services associated with the virtual IP addresses on a given virtual IP network must fail over together.
The virtual IP addresses on a given virtual IP network must go unused, thereby wasting a portion of the available address space.
Neither of these situations is desirable. Fortunately, the use of host masks remedies both.