Just had to weigh in on this issue Just look at SANS or CERT, IETF et al and their recommendations for filtering i.e. egress filters should only allow your registered address space and/or networks which you allow to transit your network and ingress filters which filter ALL RFC1918 traffic at your network border. See RFC's 2267,2827 & 3013 Ingress/egress filtering is simply good operational practice which makes the internet a more friendly place