Some ISPs do MAC-address port locking or/and use DHCP to assign IP-addresses to clients' network interfaces. Assuming you don't wanna bother calling them, what are your options then? If
ifconfig's is the only one, where should it be called from so early during boot time that no Ethernet frame would ever went to wire with its original MAC-address?
Alas, there's no 'mac-address' tag yet and you need at least 300 reps to create one.Thanks to @gentmatt, now we have one. ;-)