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:
ifconfig_fxp0_alias0="inet xxx.xxx.xxx.xxx netmask xxx.xxx.xxx.xxx"
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.
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 22.214.171.124 network with a netmask of 255.255.255.240. We want the system to appear at 10.1.1.1 through 10.1.1.5 and at 126.96.36.199 through 188.8.131.52.
The following entries configure the adapter correctly for this arrangement:
ifconfig_fxp0="inet 10.1.1.1 netmask 255.255.255.0" ifconfig_fxp0_alias0="inet 10.1.1.2 netmask 255.255.255.255" ifconfig_fxp0_alias1="inet 10.1.1.3 netmask 255.255.255.255" ifconfig_fxp0_alias2="inet 10.1.1.4 netmask 255.255.255.255" ifconfig_fxp0_alias3="inet 10.1.1.5 netmask 255.255.255.255" ifconfig_fxp0_alias4="inet 184.108.40.206 netmask 255.255.255.240" ifconfig_fxp0_alias5="inet 220.127.116.11 netmask 255.255.255.255" ifconfig_fxp0_alias6="inet 18.104.22.168 netmask 255.255.255.255" ifconfig_fxp0_alias7="inet 22.214.171.124 netmask 255.255.255.255"
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