How do I filter traffic going to two separate server?

I have two web server behind a CISCO 3600
router. my dsn record shows that one server or
host address is x.x.x.x and the other is
y.y.y.y. The router’s external interface is
z.z.z.z.

How do I distinguish what www or any port for
that mater belongs to what machine behind the
cisco router?