When Do We Use NAT?
Similar to Classless Inter-Domain Routing (CIDR), the original intention for NAT was to slow
the depletion of available IP address space by allowing many private IP addresses to be represented
by some smaller number of public IP addresses.
Since then, it has been discovered that NAT is also a useful tool for network migrations and
mergers, server load sharing, and virtual server creation. So in this chapter, I’ll describe the
basics of NAT functionality and the terminology common to NAT.
At times, NAT really decreases the overwhelming amount of public IP addresses required
in your networking environment. And NAT really comes in handy when two companies that
have duplicate internal addressing schemes merge. NAT is also great to have around when an
organization changes its Internet service provider (ISP) and the networking manager doesn’t
want the hassle of changing the internal address scheme.
Here are some situations when it’s best to have NAT on your side:
You need to connect to the Internet, and your hosts don’t have globally unique IP
addresses.
You change to a new ISP that requires you to renumber your network.
You need to merge two intranets with duplicate addresses.
You typically use NAT on a border router. For an illustration of this, see Figure 8.1.
Now you may be thinking, “NAT’s totally cool. It’s the grooviest, greatest network gadget,
and I just gotta have it.” Well, hang on a minute. There are truly some serious snags related
to NAT use. Oh, don’t get me wrong—it really can save you sometimes, but it has a dark side
you need to know about, too. For some of the pros and cons linked to using NAT, check out
Table 8.1.