The illustration has three areas bordered by dotted lines and arranged 
side-by-side. From left to right, they are labelled "Client Tier", 
"Middle Tier", and "Database Tier".

A box labelled "Internet" extends from the Client Tier to the Middle 
Tier. Inside the Internet box and completely inside the Client Tier 
is a box labelled "Browser".

In the Middle Tier area, the Internet box is connected to a box 
labelled "Firewall" by a double-headed arrow labelled "z1". The Firewall 
box is connected to a box labelled "DMZ" by a double-headed arrow 
labelled "z2". Completely inside the DMZ box are two more boxes 
labelled "Webcache" and "Non-sensitive Application server".

Still in the Middle Tier area, the Firewall box is connected to a box 
labelled "Intranet" by a double-headed arrow labelled "z3". The Intranet 
box extends from the Middle Tier area to the Database Tier area. Inside 
the Intranet box and completely inside the Middle Tier is a box labelled 
"Sensitive Application server".

In the Database Tier area are three more boxes, labelled "SQL*NET", 
"Database machine 1", and "Database machine 2". Unlabelled double-headed 
arrows connect the SQL*NET box to each database box and to the Sensitive 
Application server box.