The following post will explain one of the recommended method of filtering unwanted traffic from the internet to the internal network.
Most administrators filter RFC-1918 traversing from the internet to internal networks, while they are allowing a list of bogons prefixes which is defined in RFC-3330. These addresses are _not_ publically assigned, therefore should not see them as source IP destined to your internal network. Furthermore, it is a best practice from a security prospective to filter these ranges in case you are targeted with a spoofing attack.
As a reference to this post, please check RFC-3330 which contains all the prefixes in question.
The following configuration example shows RFC-3330 filtering on a Cisco ASA Firewall.
CREATE ACCESSLIST, where the ACL name INTERNET define OUTSIDE interface.