Selecting IP Addresses and Netmasks

When you configure the network, the appliance must be able to select a unique interface to send an outgoing packet. This requirement drives some of the decisions regarding IP address and netmask selection for the Ethernet interfaces. The rule is that only one interface can be on a single network (as determined through the applications of netmasks to the IP addresses of the interfaces).

An IP address identifies a physical interface on any given network. A physical Ethernet interface can have more than one IP address for which it accepts packets. An Ethernet interface that has more than one IP address can send packets over that interface with any one of the IP addresses as the source address in the packet. This property is used in implementing Virtual Gateway technology.

The purpose of a netmask is to divide an IP address into a network address and a host address. The network address can be thought of as the network part (the bits matching the netmask) of the IP address. The host address is the remaining bits of the IP address. The number of bits in a four octet address that are significant are sometimes expressed in Classless Inter-Domain Routing (CIDR) style. This is a slash followed by the number of bits (1-32).

A netmask can be expressed in this way by simply counting the ones in binary, so 255.255.255.0 becomes “ /24 ” and 255.255.240.0 becomes “ /20. ”