two default routes

[Jon Falconer]

I have two ISP connections, a 45Mb and a 6Mb. Depending on what block of
local addresses a packet is coming from will determine which ISP I want to
send the packet out. In essence the default route used for a packet
depends on its source address (for traffic leaving our campus.) Can
someone tell me what package I should read up on (ip,ipf,ipfw,other)? or
if I should just do this with a real router and not FreeBSD?

Thanks for your insights,

Jon