 The topic of this lecture is firewalls and network address translators. We often think about the Internet as a mean that allows users to communicate with each other. This works great if you need to access your favorite service. A little less so if this means that attackers or malicious content can also reach you. In this lecture we will investigate mechanisms for separating a network or a portion of it from the outside world, like for example from the open Internet. This week we will explore the following topics. After this introduction we will talk about firewalls, identifying their major characteristics and investigating how they works. Then we will move on to network address translator, giving the basic concept behind their operation, but also discussing their role in a security setting. And finally we will summarize.