There are four basic pitfalls of NAT that everyone should learn. Understand these pitfalls and you will be well-equipped to handle the NAT scenarios that you'll no doubt face:
- NAT can be there even when you don't know about it. The Internet does not have to be involved.
- Any two techniques to defeat NAT used together will cancel each other out.
- Some devices use a SIP ALG (Application Layer Gateway) to defeat NAT. They even do that without telling you.
- NAT correction techniques (like ALG) can falsely identify a situation and actually make things even worse.
Become familiar with these pitfalls. They are referenced frequently throughout this chapter.
Let's discuss each of these in more detail:
- NAT can be there even when you don't know it. The Internet does not have to be involved: If you are using home Internet service from your cable or telephone company, or even in some cases a business-class service, they may on occasion use NAT to put all of their customers in a separate...