A well-designed perimeter network (the part or parts of your internal network that has direct contact with the outside world ? e.g., the Internet) can prevent entire classes of attacks from even reaching protected servers. Equally important, it can prevent a compromised system on your network from being used to attack other systems. Secure network design is therefore a key element in risk management and containment.
But what constitutes a "well-designed" perimeter network? Since that's where firewalls go, you might be tempted to think that a well-configured firewall equals a secure perimeter, but there's a bit more to it than that. In fact, there's more than one "right" way to design the perimeter, and this chapter describes several. One simple concept, however, drives all good perimeter network designs: systems that are at a relatively high risk of being compromised should be segregated from the rest of the network. Such segregation is, of course, best achieved (enforced) by firewalls and other network-access control devices.
This chapter, then, is about creating network topologies that isolate your publicly accessible servers from your private systems while still providing those public systems some level of protection. This isn't a chapter about how to pull Ethernet cable or even about how to configure firewalls; the latter, in particular, is a complicated subject worthy of its own book (there are many, in fact). But it should give you a start in deciding where to put your servers before you go to the trouble of building them.
By the way, whenever possible, the security of an Internet-connected "perimeter" network should be designed and implemented before any servers are connected to it. It can be extremely difficult and disruptive to change a network's architecture while that network is in use. If you think of building a server as similar to building a house, then network design can be considered analogous to urban planning. The latter really must precede the former.