Attemping to add an IPv6 addres on an interface when the tools dont support it can trash the main address without giving any warning to the user that this is what has happened. # ifconfig eth0:2 192.168.5.5 netmask 255.255.255.0 # ifconfig eth0:2 eth0:2 Link encap:Ethernet HWaddr 00:60:08:0B:C5:44 inet addr:192.168.5.5 Bcast:192.168.5.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:10 Base address:0xff00 # ifconfig eth0:2 add 2001:618:1000::1/64 2001:618:1000::1: Unknown host Usage: ... # ifconfig eth0:2 eth0:2 Link encap:Ethernet HWaddr 00:60:08:0B:C5:44 inet addr:157.59.20.46 Bcast:157.59.255.255 Mask:0.0.0.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Interrupt:10 Base address:0xff00 Even though the user is given an unknown host message and a usage message, behind the scenes ifconfig has gone ahead and trashed the IPv4 addresss on the interface. (note the changing IPv4 address in the above cut and paste) This is with kernel 2.2.12-20 and net-tools-1.53-1