Introduction to NAT

- Network Address Translation, or NAT, was originally intended to slow the depletion of available IP address space by allowing multiple private IP addresses to be represented by a much small number of public IP addresses. Since then, it's been discovered that NAT is also a useful tool for network migrations, mergers, server load sharing and creating virtual servers. So in this chapter, I'm going to describe the basics of NAT functionality and the terminology common to NAT.

The Cisco objectives for NAT are very straight forward. You have hosts on your inside corporate network using RFC 1918 addresses and you need to allow those hosts access to the internet by configuring NAT translations. Because we'll be using ACL's in our NAT configurations, it's important that you're really comfortable with the skills you learned in the previous security video, before proceeding with this one. Because NAT decreases the overwhelming amount of public IP addresses required in a networking environment, it comes in handy when you need to connect to the internet and you're hosts don't have globally unique IP addresses.

NAT is a useful tool when two companies that have duplicate internal addressing schemes merge. Since each one can be maintained without any conflicting IP addresses. NAT is also a great tool to use when an organization changes its internet service provider, but the network manager needs to avoid the hassle of changing the internal addressing scheme. On one hand, NAT has a number of benefits. It conserves legally registered addresses. It remedies address overlap events.

Increases flexibility when connecting to the internet and it eliminates address renumbering as the network evolves. On the other hand, Network Address Translation also has some serious snags that you need to understand too. Translation also results in switching path delays. It causes loss of end-to-end IP traceability. Certain applications won't function with NAT enabled. And it complicates tunneling protocols, such as IPsec, because NAT modifies the values in the header.

NAT is truly a life-saver. But as you can see, NAT has a bit of a dark side.