IP Spoofing Question

in Security+
I was watching the CBT nuggets security+ videos where the presenter mentioned that a way to mitigate IP spoofing was to use private address space or RFC 1918 for your internal network. I do not understand though why can an attacker not spoof a private IP address?
Comments
one of the best ways to deal with it is to use firewalls and make sure they can do ip spoofing checks. set the external facing interface to never allow an ip from one of the priv ranges in, and not allow any other ip range that is behind (internal) in from the external side.
Hmm, that's not quite true. Routers won't drop those packets out of the box. It is something you have to configure. Imagine if your internal router was dropping all packets with a private IP in the header, that might cause some problem
Prevent IP spoofing with the Cisco IOS
Aside from other helpful links, this link may also help understand the concept: IP Spoofing: An Introduction | Symantec Connect