How is it possible to send identical or unfragmented IP packets?

I was just wondering thinking that this might be a security issue with the new router that I got. It works really good.It's a D-link with a built in firewall.

I guess this is the only vulnerability in my router. I have heard so far. I was tottaly complexed on how someone might do this. Thinking that if this is the only vulnerability that I have maybe there is a way to alter my router using the computer.So it would make it perfect.