The following sequence illustrates...
ifconfig eth0:3 192.168.5.1
ifconfig eth0:4 192.168.6.1
(at this point both 192.168.5.1 and 192.168.6.1 respond to ping)
Now the command:
ifconfig eth0:3 down
causes EVERY alias device associated with eth0, including eth0 itself (?!!),
to be configured down. This behavior is unique to recent 2.1.7? kernels.
People would be well advised to avoid IP aliasing if they also have
a need to to shut down the driver for a single specific alias interface.
In some cases the problem may be cirumvented by ifconfig'ing the ip alias
that would otherwise be down'ed to an invalid ip address...
(i.e. ifconfig eth0:3 10.99.99.1 rather than ifconfig eth0:3 down).