Virtual IP Addresses
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5.2.2 Unlimited Mobility
Unlike physical IP addresses which are limited in their mobility, virtual IP addresses are highly
mobile. The degree of mobility is determined by the number of servers that an IP address on a
specific server could be moved to. In other words, if you choose a physical IP address as an IP
address of a network resource, you are limiting the set of potential servers to which this resource
could be transparently failed-over to.
If you choose a virtual IP address, the set of servers that the resource could be transparently moved
to is potentially unlimited. This is due to the nature of virtual IP addresses; they are not bound to a
physical wire and, as a result, carry their virtual network to wherever they are moved. Again, there is
an implicit assumption here that the location of a virtual IP address, wherever it be, is advertised to
the owning server through some routing protocol. The ability to move an IP address across different
machines becomes particularly important when it is required to transparently move or
fail over
a
network resource that is identified by an IP address (which could be a shared volume or a mission-
critical service) to another server on another network.
This unlimited mobility of virtual IP addresses is an advantage to network administrators, offering
them more ease of manageability and greatly minimizing network reorganization overhead. For
network administrators, shuffling services between different IP networks is the rule rather than the
exception. The need often arises to move a machine hosting a particular service to some other IP
network, or to move a service hosted on a particular machine to be rehosted on some other machine
connected to a different IP network. If the service is hosted on a physical IP address,
accommodating these changes involves rehosting the service on a different IP address pulled out
from the new network and appropriately changing the DNS entry for the service to point to the new
IP address. However, if the service is hosted on a virtual IP address, the necessity of changing the
DNS entries for the service is eliminated.
5.3 Other Added Features
This section contains the following subsections:
Section 5.3.1, “Support for Host Mask,” on page 77
Section 5.3.2, “Source Address Selection for Outbound Connections,” on page 77
5.3.1 Support for Host Mask
Virtual boards support configuring virtual IP addresses with a host mask. This results in a single
address being used rather than an entire subnet. See
Section 5.4, “Reducing the Consumption of
Additional IP Addresses,” on page 78
.
5.3.2 Source Address Selection for Outbound Connections
Full resilience of connections to interface failures can be ensured only when the connections are
established between machines using virtual IP addresses as end point addresses. This means an
application that initiates outbound connections to a virtual IP address should also preferably use a
virtual IP address as its local end point address.
This isn’t difficult if the application binds its local socket end point address with a virtual IP address.
But there are some legacy applications that bind their sockets to a wildcard address (such as 0.0.0.0).
When these applications initiate an outbound connection to other machines, TCP/IP chooses the
