Rudi Ahlers wrote:
The fact is, you can use a Linux firwall with a single NIC, as long as you use different IP subnets and strong iptables rules to filter traffic properly between the 2 subnets.
another scenarion where this is used more and more these days is with virtualization, where you won't have different NIC's for each virtual server on the same physical server. The only way to firewall that traffic is to use iptables and VLAN's. And many many hosting companies use virtual hosting for their clients.
This type of setup has many caveats any it is best for the sake of newbies/noobs to say it is doable but dangerous. I know you will agree with me on this.
Those well versed in networking (should) already know all about securing this schema, and can do as they like.
Ljubomir