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We have touched on and
worked with IP addresses

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in many, many Nuggets throughout
these modules and courses.

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And so in this Nugget, I
want to go over some basics

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with you regarding IPv4.

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Number one, an IPv4 address
is 32 bits in length.

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And instead of writing it out
in a bunch of ones and zeros,

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et cetera, et cetera, et
cetera, what we do as humans,

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we represent them as
four decimal numbers.

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Like 192, 168, 1, 102.

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And then we put periods
between those numbers just

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to separate where the
numbers start and stop.

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So we represent it as
decimal, and behind the scenes

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it's binary.

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Now a part of this IP
address that server two has,

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right here, represents
the actual network ID.

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And a part of that IP address
represents the actual host

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portion.

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And the way we determine
that is using the mask.

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And what the mask
does is simple.

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It simply identifies where
the network portion stops,

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and where the host
portion starts.

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For example, in our case
right here, it's right here.

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And the way it does that,
it represents how many bits

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are being used by the network.

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So slash 24 means that the
first 24 bits are the network,

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then the remaining 8 bits
over here on the right

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represent the host
addressing on that network.

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Now for the purposes
of unicast routing,

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we have three
classes of addresses.

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Class A, B, and C. And it's
really easy to identify a class

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A from a class B from
a class C. All we do

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is simply look at the very
first number, the first octet

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before that first period.

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And for a class A,
that very first octet

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is going to be
between 1 and 126.

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There's also 127, which
is also a Class A address,

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but it's reserved for
loopback purposes.

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So as far as routable
class A addresses,

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the first octet would
be between 1 and 126.

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For a class B address,
it's between 128 and 191.

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And for a class C, it's
192 through to 223.

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And the question
may come up, well,

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Keith, why do we even care
about class A, B, or C?

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And today, we pretty
much don't, because we

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use whatever masks we need to.

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However, there are some default
masks, and here they are.

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A default mask for a class
A address is 8 bits, meaning

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the first 8 bits are the
network, and the remaining

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three octets, or the
remaining 24 bits,

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are used for host addressing.

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With a class B address,
the default mask is 16.

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And with a class C address,
the default mask is 24.

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And we don't have to
use those conventions,

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but those are the defaults.

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Another thing that's
important to be aware of

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is that there are some private
address space, based on an RFC.

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It's RFC, that's a Request
For Comments number 1918.

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And in RFC 1918 18, it
allocates some address space

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from each of the classes that
can be used inside companies.

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And when the companies use
those addresses inside,

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they don't have to
tell anybody about it.

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They don't have to register,
or request addresses,

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they can just go
ahead and use them.

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In the RFC 1918
addresses, any IP

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address that begins with 10--

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and I'm going to
say 10, anything.

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So we have an example
of one right here.

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We're using a 10.0.0 network
with this last 24-bit mask.

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And that's an example of
an RFC private address

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in the class A range.

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And in RFC 1918 for
a class B address,

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there's a 172.16 through
31, dot anything.

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And for class C, there's a
192 dot 168 dot anything.

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And that's the RFC
1918 private address

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space in the class C range.

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Now, one thing about
these RFC 1918 addresses

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is that they're not
routable over the internet.

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So service providers
are going to knock those

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off right at the edges.

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So if we're using a private
address, like this one right

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here, 192.168.1, which is
an example of the RFC 1918

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private address space,
what we would have to do

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is do network
address translation

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before those packets are sent
out to the public internet.

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Now, one of the challenges
that comes up with IPv4

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is the concept of subnets.

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Now, the idea of subnets
isn't too tricky.

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It's taking one network--

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for example, the 10 network--

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with the default
mask of slash 8.

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That's, again, from the RFC
1918 private address space.

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And then carving it up
into multiple sub networks.

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And to understand what's
really going on there,

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it takes a little bit
of time with binary

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to look at what's going
on behind the scenes.

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So regarding subnets,
and how they're created,

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and how they work,
I've got a few videos

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that will walk you through
some of the basics of binary

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and how to do custom subnetting.

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Another challenge that comes
up is routing, and reverse

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engineering with IPv4.

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Now, from routing perspective,
when a Windows Server gets

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a packet, it's going to forward
it based on its routing table.

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And if routing is
enabled, it's also

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willing to forward other
devices' traffic through it.

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But in either case, it's going
to refer to its routing table

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when it's making a
routing decision.

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And so as we look at
this routing table,

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the rule is we're
looking for the longest

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match in the routing table.

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That's the route
we're going to use.

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So as I look at
the entries here,

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I don't see anything
that start with 23.

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And as a result, it's going
to go to the default route.

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I call it the
Obi-wan Kenobi route.

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Help us, Obi-wan,
you're our only hope.

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And so in this example, you use
the default gateway 192.168.1.1

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to forward that packet.

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Another challenge that
comes up is the need

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to be able to do
reverse engineering.

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For example, let's
say we saw a computer,

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we looked at its IP
address, and let's say

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the IP address was 23.17.204.98,
and it had a mask of 19,

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meaning the first 19
bits of the IPv4 address

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are representing the network.

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And the rest is representing
the host address.

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A good question is, what
exactly is the network--

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the subnet-- that this
host is connected to?

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And the ability to take a host
address like this one with

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a mask and reverse engineer the
network ID-- the subnet ID--

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also requires, my friend,
a pretty good understanding

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of binary and subnetting.

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And have no fear, that's what
the next several videos is

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going to walk us
through together.

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So until then, I hope this
has been informative for you.

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And I'd like to thank
you for viewing.

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