Which guide are you using for your CCNA preparation? I will compare the famous book by Mr. Todd Lammle with Mr. Richard Deal. Everyone suggests Todd lammle for sure and I think it depends how far one can absolutely love it. I did not like it as much as everyone else.

Well here are the things you should know about Todd's guide:

1.He talks about stuff you might have never heard of, yet he promises to tell you about it later in the book  and he tells that in a friendly way, this is frustrating.
2.The book covers over and above CCNA objectives, its good in a way but if our priority is becoming CCNA, those extra topics would kill your time.
3.His style of teaching is OK but not so good on few topics, actually waste of time and energy.
4.For configurations the book rocks, thats for sure. He uses a topology throughout the guide and implements different configurations like RIP, Eigrp, OSPF, interface configurations, IOS etc etc.
5.The book doesn't cover wireless topic effectively, you might get 3 or 4 questions from wireless in the test.
6.Whats beyond CCNA in this book: CNA for switches, IP telephone configs etc. If you've decided to use Todd Lammle's book, its very important to check his website for updates - www.lammle.com

What about Mr. Richard Deal's guide for CCNA?

1.The book is not so famous as Todd's but absolutely rocks!!!
2.You'd just read it like a story, yet the guide does not use informal way of explaining.
3.This guide is near to perfection, starts from the very basics of what a "network" is. So if you're completely new to networking, you will definitely appreciate this guide.
4.There are "Exam Watch" notes in between and they're direct questions from CCNA.
5.You complete a chapter, complete the "inside exam" section and take the "two minute drill"and then take the test, you will nail the subject and will not forget.
6.Configurations are not as good as Todd's but definitely NOT bad.

What now?
If you're confused which one to go for, go for Richard Deal, you won't regret it and if you have enough time you can through the configurations from Todd's guide. Like, after you're done with RIP, EIGRP, you take a look at the configuration section in Todd's guide. But this is redundant cos your labs are already covered in Richard's guide. And use GNS3 for your lab practice.

If you want to do a mixed study from both guides, you might wanna study these topics from Richard's guide for sure to be on safe side:

• Layer 2 Technologies (all about cables and layer 2 protocols)
• VLANs and Trunks(dot1q, ISL, EtherChannel etc)
• Switches and Redundancy (STP, PVSTP+, RSTP etc)
• WAN - Frame Relay, VPN
• IPV6
• NAT
• Wireless

Reminding again, the 'Exam Watch' notes in Richard's guide are answers to direct questions from CCNA.
And for the rest Todd's guide did pretty good including:

• OSI Model but take a look at Richard's on Data Link Layer (LLC and MAC) 
• Subnetting, VLSM, troubleshooting TCP/IP
• IOS and SDM
• Managing Cisco Internetwork 
• IP Routing - RIP, EIGRP, OSPF
• Security 

What is NAT?
Network Address Translation translates your local or private IP addresses to public IP addresses so you can communicate with the internet world.
Why NAT? Because its expensive to buy public IP addresses to each and every computer in a network.

Types of NAT:
Static NAT: This is like one-to-one i.e. your one particular private ip address is always translated to one dedicated public ip address.
Dynamic NAT: This is like many-to-many. Your list of private ip addresses are translated to a pool of public addresses, a public ip dynamically picked from that pool for your private ip addresses.
PAT-Port Address Translation: This is like one to many. Your list of private ip addresses are translated to one public ip address. This uses source port no. of your local network to differentiate individual hosts or computers in your local network. This is also called NAT Overload.

Lab:
Local router:
f0/0 - desktop
f0/1 - vpcs1
s1/0 - border router

Border router:
s1/0 - local router
s1/1 - isp router

Isp router:
s1/1 - border router and f0/0 - vpcs2

• Desktop( MS Loop back Adapter) gets 10.0.0.2   255.255.255.252, GW: 10.0.0.1, DNS:  10.0.0.1
• We will use EIGRP in Local and Border routers for routing protocol, so that desktop and vpcs can communicate with Border router via Local router. We will configure them both routers in Autonomous system no 10 so they become neighbours and exchange topology database.

• Border router should be able to communicate with both public and private networks. With Eigrp, border router is already communicating with our local networks. In real world, all local computers connect to border router and border router knows how to get to ISP router, these settings are given by ISP provider. For this lab, we will need to configure border router ourselves, it needs to know the routes. We can use any routing protocol but because we only have one network 99.0.1.0 to be included in the routing table, since 99.0.0.0 is directly connected, lets use static routing.

• I also used 'ip default-network' command so EIGRP can adveterise it to local router.
• With all the above setup we should be able to succefully ping within our local network and within our public i.e. 99.0.0.0 and 99.0.1.0 network. But we can not ping from local network to public network and thats where NAT comes into picture. NAT will translate our local ip addresses to public ip address so we can communicate with our public networks. We are going to implement NAT overload or PAT in our border router.

• First line creates a pool called 'test' with a range of public addresses. In this case only 99.0.0.1 because we're implementing NAT overload i.e. many-to-one. However, we can use more than one public IP.
• Second line creates a list of our local addresses identified by no. '10' with 'permit' option.
• Third line tells NAT to translate inside addresses that are specified in list no. 10 to addresses in pool named 'test', the 'overload' tells NAT to use port no.'s to identify a particular host in the local network.
• Now we're left configuring interfaces as belonging to inside network or outside network in our border router. In this lab, s1/0 of border router belongs to inside network and s1/1 to the outside.

Verification:

References:
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml