Secure Internet Connection Behind NAT Router

You can connect to internet securely behind NATed router as firewall.
Requirement:
- Cisco Router with 1 LAN Interface (Ethernet) and 1 WAN Interface (Serial).
- Internet connection.
hostname InternetRouter
!
enable password 123
!
no ip name-server
!
interface Ethernet 0
description connected to EthernetLAN
ip address 192.168.1.1 255.255.255.0
ip nat inside
!
interface Serial 0
description connected to Internet
ip address 123.123.123.1 255.255.255.252
ip nat outside
!
no access-list 1
access-list 1 permit 192.168.1.0 0.0.0.255
!
ip nat translation timeout 86400
ip nat translation tcp-timeout 86400
ip nat translation udp-timeout 300
ip nat translation dns-timeout 60
ip nat translation finrst-timeout 60
ip nat inside source list 1 interface Serial 0 overload
!
ip route 0.0.0.0 0.0.0.0 Serial 0
!
line console 0
exec-timeout 0 0
password 123
login
!
line vty 0 4
password 123
login
!
end