As Sven-Hendrik said, you need to use ifconfig $interface up and ifconfig $interface down. If you really need ifup and ifdown, put this in your .bashrc:
alias 'ifup eth0'='ifconfig eth0 up' alias 'ifdown eth0'='ifconfig eth0 down" It's likely that you could replace hardcoding eth0 with using something like $2, but it's 4AM and i'm too lazy to write up a example for that.
