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Okay, so as the Internet Assigned
Numbers Authority got
involved here,
and they started organizing and
controlling these IP networks and
IP addresses,
and dividing them into classes
like A, B, and C - and then D and
E but we're not really going to
focus on D and E addresses -
they said, Well, here's what
we're going to do.
The majority of these
IP networks,
like the 146 network or the
99 network,
we will make those as what we
call public networks which is a
network that somebody can
rent or lease,
and then that network will be
unique just to
their organization.
And then all the service
providers throughout the entire
world will know that that network
sits over here in this company -
it's owned by that company
So that's called a
public IP address.
It's something that's leased
to a corporation.
Well then the Internet Assigned
Numbers Authority set aside a few
networks as private or
unregistered networks.
These are networks that
nobody controls.
Networks that anybody can use,
but you could only use them
within your company.
In other words,
it's a networks that's not
allowed to send packets into or
receive packets from the global
public internet because if I sent
a packet into the internet going
to a private destination,
there's no way that service
providers and routers within the
internet would know how to get
that packet to where it needs to
go because that private address
could be used by thousands or
tens of thousands of companies
all using the exact same address.
So private addresses are just
used within your company.
For example,
let's say that I had within my
enterprise a section of my
network over here that had a
whole bunch of private file
servers on it.
So I want all the employees
within my network - within my
company - to reach this private
broadcast domain over here with
these private file servers.
But I never will ever need anyone
from the outside world to get to
those servers.
They're only private for my own
internal use.
Well the broadcast domain -
the VLAN,
the Ethernet segment where those
file servers live - I could
address that with a private
network number,
and as long as the routers within
my company know how to reach it
my own internal employees could
get to those servers but the rest
of the world it's invisible
to them.
That private network is not
advertised out to the rest
of the world.
So if you're ever curious you can
Google RFC 1918,
that gives more gory detail about
these private networks.
And these are the range
of addresses,
and for Cisco's certification,
this is definitely something
you're going to want to
have memorized.
So each of our classes,
Class A, B, and C,
some of the address space was
reserved for these
private networks.
You can see here the
Class A space,
anything beginning with the
number 10 is a private network.
In Class B,
anything beginning with 172.16
through 172.31.
Remember, like we saw
with the whiteboard,
with that number 172 in Class B,
there are 256 networks available
that start with 172.
There's 172.0 up to 172.255.
And we see that a handful of them
here in the middle .16 through
.31 were reserved for
private usage.
And then the Class C space,
192.168.anything is reserved for
private usage.
Now, it's kind of interesting
here if I show you a - actually
not this - if I show you my DOS
prompt for a moment,
and I look at what IP address I
have right now,
you'll see I've got nothing but
private networks on here.
192, that's private,
and if I look at my WIFI adapter
that's 10,
that's private.
So you might be wondering,
Well, wait a second.
How can my laptop reach the
outside world at all if the only
IP addresses it has are private
IP addresses?
Well, the reason it can reach
the outside world is typically
in a network that utilizes
private addresses.
You'll have at least one device -
typically a router - that has a
public IP address on it.
It has a network on it that was
bought and paid for that
was leased,
and that router is actually
doing translation.
This is called network address
translation where my laptop sends
a packet to the Internet,
like CNN.com or Google.com
or whatever,
when that packet reaches my
router here within my company the
source address of that packet is
my private address - in this case
10.something, 10.7.1.61.
Well, when it reaches the router
the router says,
Okay, clearly I cannot forward
that packet into the Internet as
it stands because it's got a
private source address, so the
router actually strips off that
source address and replaces it
with a public IP address.
So when it leaves the router it
looks like it came from a
public network,
and that's called network
address translation.
That is a feature that you do
need to know for the CCNA.
And public addresses are pretty
much everything else.
So in the Class A, B, and C space
- anything that wasn't private
that we just saw - falls into the
public space.
Not entirely true.
We know that the 127 network
that's not public or private.
The 127 we said that was reserved
for loopback testing.
There's a handful of other
networks as well that sort of
have special meanings.
For example,
if I go back to my DOS prompt
right here for a second-- look at
my Ethernet adapter.
My Ethernet adapter has a network
beginning with 169.
Now you might look at that
and say,
Okay, well we just learned that
in binary the number 169 begins
with 10.
The 128 bit is on,
the 64-bit is off.
So that is a Class B network.
Well that is true.
It falls in the Class B space,
but that is neither a private nor
a public network.
That is a special network that's
called an automatically
provisioned IP address.
Basically on systems that utilize
DHCP, the Dynamic Host
Configuration Protocol,
if a system like pretty much
everything these days:
laptops, PCs,
tablets, smartphones,
if something utilizes DHCP to
dynamically discover what IP
address it should use,
sometimes DHCP fails.
Sometimes, for example,
like in this case my laptop says,
okay, I'm connected to a cable,
which it is,
my laptop has an Ethernet cable
plugged in.
And it attempted to dynamically
discover its IPv4 network via
DHCP and it failed.
And when DHCP fails,
it automatically provisioned
itself with this,
169.254.something.
So usually when you see this,
anything beginning with 169,
that's actually bad.
You don't want to see that.
That means that DHCP failed and
you are not able to get yourself
a real usable IP address,
but it had to put something in
this 32-bit number field,
so it shows this special reserved
number of 169.254,
which is called an automatically
provisioned IP address.
Not usually a good thing.
You don't really want to
see that.
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