- [Presenter] While IPV4 addressing
is what currently connects most of networks
and carries a large portion of the information
across the internet,
globally routable IPv4 address space has become scarce.
We've already breached the exhaustion threshold
and internet registries have run out of IPv4 addressing
that can be allocated to various organizations.
This isn't a new issue.
In fact it was in the 1980's and early 90's
that mitigations began being put into place.
Two network level stop gaps were
private addressing and network address translation.
RFC1918 introduced three ranges of IP addressing
considered to be private addressing.
The concept of private means that these are not
IP addresses that are reachable
on the globally routed internet table.
In fact, they are only reachable
within a single organization.
This means that now using private addressing
networks of tens of hosts to thousands of hosts
can now be addressed without exacerbating IPv4 exhaustion.
Now that my infrastructure is privately addressed
how can I get my network to access the public internet?
The answer is network address translation.
NAT essentially allows a piece of network equipment
to map one IP address to another.
There are actually a few forms of NAT-ing
some are employed more than others.
First to discuss is static NAT-ing
or one-to-one NAT-ing.
In this example, as client 10.0.0.2 moves through the router
its IP address is translated to 100.0.0.2.
When NAT-ing in cisco routers you must establish
the concept of inside and outside.
Generally the inside interface is something facing
the LAN while the outside is something that faces
the internet.
Cisco then calls the LAN private addressing the inside
local addressing, while the public IP is called
the inside global address.
Usually in my mind I think inside local means private
and inside global means public.
This addressing doesn't always have to be private
to public, it can be private to private or public
to public too.
With the static NAT as shown in the diagram, when the host
moves through the router, the addressing will go local
to global but also as traffic comes into the router
from the outside interface the reverse will occur.
The inside global will be translated back to the inside
local address.
Dynamic NAT operates similar to static NAT in that
it creates a one to one relationship but it does it
dynamically via a pool of inside global public addresses.
While this NAT type is possible, it's virtually never used.
What is used in virtually every network I've configured
is NAT overload or port address translation.
This allows many privately IP hosts to translate
to a single public IP address all dynamically.
PAT utilizes port numbers to distinguish traffic flows
through their router.
As a connection moves through the router it will note
source and destination addressing as well as port numbers
used, it will then change the source address in the packet
and send it along its way.
When traffic in this flow returns the router will look up
the port and addressing combo when its NAT translation
table change the destination address in the packet
back to the original private address of the sending host
and forward it on its way.
This allows a sizeable number of hosts to all utilize
a single address to access external resources
quite efficiently.
I've not touched a network that didn't utilize private
addressing along with NAT-ing of some sort.
So pay particular attention to this chapter.