It isn't possible to use IP addresses like 10.x.y.z across the internet - those are used by companies internally. And anyone can use them. Likewise 192.168.x.y and some other ranges. But it is possible for someone outside my network to construct a packet claiming to come from such an address, and send to to my network. These are called Martian packets because packets can't actually come from there. This can be a symptom of an attack, or maybe of just a misconfigured system. Anyway, I don't want them. So I told my pix to deny them access.