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# 11.9 Virtual Hosts

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 `1`s (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`. We want the system to appear at `10.1.1.1` through `10.1.1.5` and at `202.0.75.17` through `202.0.75.20`. As noted above, only the first address in a given network range (in this case, `10.0.1.1` and `202.0.75.17`) 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:

    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 202.0.75.17 netmask 255.255.255.240"
    ifconfig_fxp0_alias5="inet 202.0.75.18 netmask 255.255.255.255"
    ifconfig_fxp0_alias6="inet 202.0.75.19 netmask 255.255.255.255"
    ifconfig_fxp0_alias7="inet 202.0.75.20 netmask 255.255.255.255"

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