Apart from that, a real firewall is SO MUCH MORE than just something that closes ports from the outside. It all comes down to what you want to protect, and how much time/effort your willing to invest. Whilst for a simple home network, a simple router with firewalling functions might suffice, it is far from being a real firewall with tighter security.

There are many different types of firewalls, that offers different types of security. Dont forget that a large risk in computer security happens also from the inside, and not only from the outside.

Cheers.