Question Regarding IP Alias

[Sashikiran Rachakonda]

Hi,i have a question regarding the IP Alias. If i bind my interface to 2
ip addresses say
		
		1) ifconfig xl0 add w.x.y.z netmask 255.255.255.0 
		2) ifconfig xl0 add p.q.r.s netmask 255.255.0.0

Is there a way that i can force the packets coming-out of this interface
to have ipSrc = p.q.r.s and not w.x.y.z. My question is is there a way
that you can tell the interface to have the IPsrc set to the one we want
to, for all packets coming out of this interface.

Thanx in Advance,
--Sashi.