  Network firewalls and routers use a rule database to decide which packets
will be allowed from one network onto another. By filtering packets the
firewalls and routers can improve security and performance. However, as the
size of the rule list increases, it becomes difficult to maintain and validate
the rules, and lookup latency may increase significantly. Ordered binary
decision diagrams (BDDs) - a compact method of representing and manipulating
boolean expressions - are a potential method of representing the rules. This
paper presents a new algorithm for representing such lists as a BDD and then
shows how the resulting boolean expression can be used to analyse rule sets.
