A very common use of FreeBSD is virtual site hosting, where one server appears to the network as many servers. This is achieved by assigning multiple network addresses to a single interface.

A given network interface has one “real” address, and may have any number of “alias” addresses. These aliases are normally added by placing alias entries in /etc/rc.conf.

An alias entry for the interface fxp0 looks like:

Note that alias entries must start with alias0 and proceed upwards in order, (for example, _alias1, _alias2, and so on). The configuration process will stop at the first missing number.

The calculation of alias netmasks is important, but fortunately quite simple. For a given interface, there must be one address which correctly represents the network's netmask. Any other addresses which fall within this network must have a netmask of all 1s (expressed as either 255.255.255.255 or 0xffffffff).

For example, consider the case where the fxp0 interface is connected to two networks: the 10.1.1.0 network with a netmask of 255.255.255.0 and the 202.0.75.16 network with a netmask of 255.255.255.240. The system is to be configured to appear in the range 10.1.1.1 through 10.1.1.5 and 202.0.75.17 through 202.0.75.20. Only the first address in a given network range should have a real netmask. All the rest (10.1.1.2 through 10.1.1.5 and 202.0.75.18 through 202.0.75.20) must be configured with a netmask of 255.255.255.255.

The following /etc/rc.conf entries configure the adapter correctly for this arrangement: