5.3 Other Added Features

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.

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 through 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. However, there are some legacy applications that bind their sockets to a wildcard address (such as When these applications initiate an outbound connection to other machines, TCP/IP chooses the outbound interface’s IP address as the local socket end point address. In order for these legacy applications to take advantage of the fault resilience provided by the virtual IP address feature, the default source address selection behavior of TCP/IP has been enhanced to accommodate the use of a virtual IP address as the source IP address. As a result, whenever a TCP or UDP application initiates an outbound connection with a wildcard source IP address, TCP/IP chooses the first bound virtual IP address as the source IP address for the connection.

This enhanced source address selection feature can be enabled or disabled globally as well as on a per-interface basis. This feature is enabled by default on all interfaces.