When you run a shell command to perform a task that requires system administrator permission, you need to put sudo
in front of it. sudo
stands for “do as super user”. You will be prompted for your password (if you run several commands in quick succession, you will only get a password prompt the first time), and you must be authorized to run command as administrator (which is the case for the initial user created during an Ubuntu installation).
sudo ifconfig eth0 hwaddr 01:23:45:67:89:ab
See the Ubuntu wiki for more information.
If adding sudo
isn't enough, edit your question to be more precise. Copy-paste the command(s) that you ran and the error messages.
sudo
EG instead ofifconfig eth0 192.168.1.5 netmask 255.255.255.0 up
for example it should besudo ifconfig eth0 192.168.1.5 netmask 255.255.255.0 up
It would also be more helpful if you added the commands you have used to your question to receive better help.