 In this lecture, we have discussed two methods to separate the network from the unsafe outside world, firewalls and network-advent translators. We have seen the basic idea behind the functioning of firewalls and investigated the most important types of firewalls, outlining the advantages and disadvantages of each one. Finally, we have reasoned about how one or more firewalls can enhance the security of a network. Similarly, as for firewalls, we have explained the functioning of the mechanism of network-advent translation. We have then seen which type of NAT exists, and most importantly, we have discussed the security implication of NATs and the reasoned about the similarity and differences between firewall and NATs.