what ifa / ifp ?

[bsenthil]

route add -net 10.1.1.1/24 0.0.0.0 -ifp fxp0

route add -net 10.1.1.1/24 0.0.0.0 -ifa fxp0   (Error ..ifa)

$ > man route....
In a change or add command where the destination and gateway are not suf-
     ficient to specify the route (as in the ISO case where several 
interfaces
     may have the same address), the -ifp or -ifa modifiers may be used to
     determine the interface or interface address.

Coule you please explain what -ifp or ifa modifiers mention...

Thanks,
Senthilkumar.