Well, this happens only if you have a variable ifconfig whose value is ifconfig and you're not root, that is
-bash: ifconfig: command not found
but I see no reason to launch a command as $variable, unless you have a script which dynamically set the name of the command and store it in a shell variable. Moreover, as already stated by SqdnGuns above, you have to be root to run ifconfig otherwise - as a regular user - it is not in your PATH.
Actually, you can run the ifconfig command as a regular user if you launch it with its absolute path:
but only to display the ethernet interface settings, not to modify the settings themselves - for which you have to acquire root privileges. Please note that in my examples above the greyed $ sign is the command prompt.