Mandrake stores it's ethernet interface configuration info in the following file:
/etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
IPADDR=64.81.64.188
NETMASK=255.255.255.0
NETWORK=64.81.64.0
BROADCAST=64.81.64.255
ONBOOT=yes
MII_NOT_SUPPORTED=no
WIRELESS_ENC_KEY=""
So, if changes made using ifconfig don't seem to persist between reboots, try editing this file.