At the end of Chapter 2, “Internet Protocols” of the Sybex
CCNA Study Guide
I provided a section called “Introduction to Network Address
Translation.” In it, I explained some basic terms, but I’ve been hearing from readers that it
doesn’t have enough information. I really have to expand on Network Address Translation
(NAT) in order to equip you with a thorough understanding of this very important topic.
So what’s new here? Well, in this update, I’m going to give you the skinny on NAT, Dynamic
NAT, and Port Address Translation (PAT)—also known as NAT Overload—in a lot more
detail. And I’m going to finish this update with an important hands-on lab so you can test your
understanding of these topics.
I’m giving you this update with the assumption that you’ve read at least
through Chapter 6, “Enhanced IGRP (EIGRP) and Open Shortest Path First
(OSPF),” of the CCNA Study Guide. Of course, it will be even better if you’ve
read the entire book!
So… When Do We Use NAT?
NAT, at times, decreases the overwhelming amount of Public IP addresses required in your net-
working environment. And NAT comes in really handy when two companies that have dupli-
cate 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 to hassle
with changing the internal address scheme.
Here’s a list of 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 require two intranets with duplicate addresses to merge.
You typically use NAT on a border router. For an illustration of this, check out Figure 1.1.
Okay—so yeah. NAT’s totally cool. It’s t