At this point it has to be distinguished between the different Linux distributions.

Gentoo:

The file can be found in etc/conf.d/net

Information can be found in the /etc/conf.d/net.example

SuSE / openSUSE

The network configuration file can be found in
/etc/sysconfig/network/ifcfg-eth0

Here the file has to be modified as follows for each additional IP :

IPADDR_AAA=xxx.xxx.xxx.xxx
NETMASK_AAA=yyy.yyy.yyy.yyy
BROADCAST_AAA=zzz.zzz.zzz.zzz

Whereas xxx/yyy/zzz stands for the following values:

xxx.xxx.xxx.xxx => IP address
yyy.yyy.yyy.yyy => net mask
zzz.zzz.zzz.zzz => broadcast address