Another example of flat structure is XHTML that USES the tag to mark the beginning of new sections rather than any explicit section grouping.

There's a good section that shows how to use iptables to mark packets, and then use Linux routing functionality to route the packets based on these marks.

For years, critics have mocked the idea of people being willing to mark their content up this way, but as we will see in the next section, they were sorely mistaken.