How Network Address Translation Works

Explanation of what Network Address Translation is, how it works and why we need it to keep the internet growing. IP version 4 and 6 are also discussed.

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in this video I'll talk about network
address translation how it works and why
we need it but first let's hear a bit of
backstory
where do these little baby IP addresses
come from anyways
back in the late seventies a couple of
collaborative researchers was thinking
about how long IP addresses should be
why is this important you have to
consider that in every data packet we
have to encode the center IP and the
receiver IP if we only reserve a few
bits to encode an IP address
we can keep packet size is small however
will also have few IP addresses to hand
out to people
if on the other hand we choose to have
lots of bits to encode an IP address
will have lots of IP addresses to hand
out but will also have huge packets
which increases the communication
overhead
so as you can see there's a bit of a
trade-off going on here
you can either have small packets or
lots of IP addresses they ended up
deciding that they go with 32 bits for
an IP address that left them with
roughly 4.2 billion IP addresses you
have to consider that back then the
internet which wasn't even called the
internet back then was just a toy for
academics it was definitely not meant
for regular people like you and me and
that's what's known today as IP version
4
now hang on a second let's do a bit of
math here we have 4.2 billion IP
addresses to hand out but we also have 7
billion people living on this planet as
of 2012 and also consider that each of
those seven billion people may want to
use multiple devices to access the
internet so yeah we have a bit of a
problem on our hands
what are we going to do about this now
because clearly having only 4.2 billion
machines on the internet is not going to
be an option we have to figure something
out
and there is a fix but it's as you'll
see a bit of a workaround let's keep it
at that there is in fact a way that we
can keep using IP version 4
and get more machines on to the internet
here's a typical home network setup
this is John's laptop this is james
laptop and this is Jane smartphone
they're all connected to the home router
that home router in its turn is
connected to the internet when you
register with your isp you get to use an
IP address that is accessible throughout
the internet and that's assigned to your
home router so in this case we have
twelve . thirteen . 14 . 15 assigned to
the home router
this is a public IP address that
theoretically anyone on the internet
consent packets to our laptops and smart
phone also get their own IP address but
instead of getting one of those 4.2
billion public IP addresses they get a
private IP address that private IP
addresses assigned to them by the home
router and is not accessible from the
internet now with these IP address look
familiar to you that's probably because
they are
this is one of two ranges that are
reserved for private networks all
addresses starting with 192 . 168 are
reserved for private networks as well as
all addresses that start with the number
10
you can use these IP addresses without
having to worry that some random web
server already uses that IP address
they can because it specifically
reserved for private networks
now let's consider the connection from
James laptop for a second
James laptop has an IP address of one
ninety two dot one sixty eight dot one .
3
here's an example scenario where James
laptop wants to know what the current
whether it's in taxes and to get this
information it will try to contact the
server at 40 . thirty . 20 . 10 to get
that information to reach this web
server change the laptop will have to go
through the home router through the
internet and finally then
arriving at the web server first change
the laptop has to send out a packet
requesting the information
it says what's the current temperature
in san antonio texas
now the message here isn't really
important the web server should know
what to do with that and we won't get
into that right now but what you should
know is that there is a source IP
address here with the source port and a
destination IP address with a
destination port in this case its port
80 which is frequently used for HTTP
traffic
now let's send this package on its way
shall we
when it arrives at the web server the
web server will process the incoming
requests and try to formulate a reply
the reply packet will be addressed to
the center but in this case because it
was sent from 192 168 1 . 3
it will not be reachable because it's a
private IP address
so what do we do now let's rewind that
tape for just a second
James laptop sends out a packet
requesting the current temperature and
san antonio taxes it gets sent along to
the home router but instead of sending
it right over the Internet
the home router does something that's a
little bit sneaky when that packet
re-emerges you'll notice that the source
IP address has been changed
this is not the only thing that the home
router does in addition it also creates
an entry in the net forwarding table now
we'll talk about net forwarding table
again in just a second but for now all
you have to remember is that this thing
allows us to know which packets are to
be sent and James laptop when they come
in and which packets are to be sent to
that smartphone or John's laptop
moving right along the packet travels
over the internet and arrives at the web
server the web server will now create a
reply packet and will notice that the
destination IP address is no longer
private
no instead
sent to the public IP address of our
home router on that specific port now
that our home router has received the
response it's time for another tiny lie
it creates a packet that looks as if it
was a dress from the web server directly
to James laptop which is not what
happened
but that's all James laptop needs to
know because it it doesn't care about
how it got there
it just wants to get the packet and get
this over with
but how does it know where to send that
package just received the answer is in
the net forwarding table
we just received that packet on poor 24
604
and that means if we look at our net
forwarding table that we have to change
the IP address on the private side to
one ninety two dot one sixty eight dot
one not three on that specific port over
there in the forwarding table
James laptop receives the packet and the
rest is history
James laptop doesn't have to care about
any of that NAT stuff
the home router completely handles it
for James laptop and it is completely
transparent so James laptop doesn't have
to worry about public and private IP
addresses know this is something the
router fixes and James laptop doesn't
have to worry about it
of course ideally every computer would
have its own public IP address which is
why we're slowly but well actually just
slowly moving over to IP version 6
remember how I p version four had four
point two billion IP addresses at its
disposal
well IP version 6 has a lot more and i
do mean a lot
seriously trying noting 34 and then
followed by thirty seven zeros and
that's how much ipv6 addresses we have
approximately
that is a lot if we can give every
computer its own public IP address
we won't be needing network address
translation anymore
this means that router skin once again
focus on there
the core business which is routing
packets not doing awkward switcheroo
tricks now I can hear you saying this is
nice and all but why haven't we switched
yet this IP version 4 thing is a mess we
only have four point two billion IP
addresses and that as clumsy as hell
well it's not that simple we have a lot
of Internet infrastructure that was
built up over the years and they all
work with IP version 4
so we have laptops desktops home routers
but also stuff on the ISB side and web
servers and they all work with IP
version 4 all these devices need to have
hardware and software that supports IP
version 6 before we can entirely switch
over and that is a huge task
just to give you an idea the IP version
6 standard was finalized in December of
1998 and yet here we are 14 years later
and was still running on IP version 4
you can see how this is one hell of a
task
all righty if there's anything you'd
like to see explain please do let me
know in the comments below thanks for
watching and I'll see you next time