The illustration has two areas bordered by dotted lines and arranged 
side-by-side. The left area is labelled "Three Zone Firewall 
Implementation One" and the right area is labelled "Three Zone 
Firewall Implementation Two".

In the left area is a box labelled "Firewall machine" with three 
double-headed arrows extending away from it. The arrows are labelled "z1", 
"z2", and "z3". The points at which these arrows touch the Firewall 
box are labelled "i1", "i2", and "i3" respectively.

In the right area are two boxes labelled "Firewall machine 1" and 
"Firewall machine 2". The former has a double-headed arrow labelled "z1" 
extending away from it; the point at which it touches the box is labelled 
"i1". The latter has a double-headed arrow labelled "z3" extending away 
from it; the point at which it touches the box is labelled "i2". The two 
boxes are connected to each other by a double-headed arrow labelled "z2". 
It touches the Firewall machine 1 box at "i2" and the Firewall machine 2 
box at "i1". This double-headed arrow also has another double-headed arrow 
extending away from it.