Cisco bought Linksys several years ago. Cisco is a leader in router security. They spread their tactics throughout Linksys (whose tactics were already very good), in hopes of capturing the home market.
Linksys routers today are fully NAT out of the box. They're not high and tight, but no-one's routers are. Read the directions, ancilliary info you find online, and tweak your the snot out of your router's settings until it no longer works. Then, start from scratch and do it again, until you've locked down everything you can in a systematic process until you can't lock it down any further without loosing something you'd rather not do without.
The cool thing is, you'll learn a lot in the process.