Hi Guys, I have always trusted NAT as my firewall solutions but in the back of my head I've know this is unsecure.
While I don't want to restrict my users access to the Internet, I would like to make ports hidden on the Internet and secure the Internet connection as much as possible.
Also a quick explanation of the configuration would be much appreciated.
PS. It is unfortunate and a lacking area in the CCNA to cover ip inspect rules and simple adsl configurations
Base on the configuration posted, basically you are allowing all outbound traffic, however, restrict inbound traffic to the following: RDP, SSH, mail, web, PPTP, so I would assume that either you have a public ip address range for those servers' services, or you have configured static port address redirection to your Dialer interface ip address.
If you have a static public ip address, I would recommend that instead of configuring "permit tcp any any" on specific ports, I would tighten it even down as follows (assuming your public ip address is 126.96.36.199):
permit tcp any host 188.8.131.52 eq 3389
permit tcp any host 184.108.40.206 eq 22