Add a NAT router

If all machines on a NAT network are not equally secure and trustworthy, if a machine is infected, the malware could have access to every other uninfected machine sharing the once-secure LAN. By sending ARP broadcasts to the LAN, an infected machine can determine the IP and MAC addresses of every other machine on the LAN.

This is a known LAN-side vulnerability and why many viruses and worms attempt to spread not only by scanning the internet for additional vulnerable targets, but also try to spread locally through M$ file sharing, RPC vulnerabilities, and many other well-known vulnerabilities. A case of Emmenthaler cheese with extra holes of which only some crumbs are left on the plate.

A second NAT router can be used on the internal LAN to create a second, more secure LAN.


  • Machines on the more-or-less-secure internal LAN can access the internet, but they are protected by the External NAT from most malwares.

  • Machines on the more-secure internal LAN can also access the internet, first by going out through the Internal NAT and then the External NAT.

  • Because the more-or-less-secure internal LAN is on the WAN side of the Internal NAT, machines on the more-or-less-secure internal LAN are unable to freely access the machines on the more-secure LAN behind the Internal NAT. Machines behind the Internal NAT can access the machines in the middle, but NOT the other way around.

When and where is this useful?

  • For isolating a router’s Demilitarised Zone (DMZ) network and servers.

  • Isolating an open or low-security wireless access point.

  • Protecting one ‘’high-value’’ machine from the rest of the network.

Take control of the Internal NAT router

This can be made even more secure by flashing the Internal NAT router with custom router firmware, tightening security via its firewall, and securing access to it.