In UDP multicasting, every multicast group is bound to a network interface, using the IP address of the interface. RAP documentation refers to this interface as the local interface.
Every network interface card (NIC) on your computer has an IP address associated with it. This address is assigned when the computer is configured, or after the card is installed, by means of the utility ifconfig. Typically, a computer will have just one network interface, and its IP address becomes the IP address of the host computer. If a second network interface card is added, its IP address must be assigned by a network administrator, and will be different than the host IP address.
RAP uses IPv4 multicasting, and requires IP addresses configured for the network interface to be 32-bit IPv4 addresses.
For systems with a single network interface, multicast groups will always be associated with the host IP address. In cases where more than one network interface exists, the multicast group may be associated with the IP address of the second interface. This is often the case when a network interface is dedicated to a subnet for very high speed linkage between hosts within the subnet.
When considering which local interface to use for configuring RAP systems, and there is more than one network interface card installed on your computer, it is recommended that you consult with your network administrator to determine which interface to use for RAP. RAP requires the use of an interface capable of transmitting at a minimum rate of 1 gigabit per second.
A demonstration installation of RAP that runs on a single machine may use the loopback interface as the local interface. The loopback interface (127.0.0.1 for IPv4) is not a physical device but a piece of software simulating a network interface. Any traffic that a computer program sends on the loopback interface is addressed to the same computer.