- 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.