If you're configuring IPv4 on a computer,
then you need to configure three very important addresses.
The first address is an IP address.
This is an address that uniquely identifies
this device on the network.
If you look at your IP address, and it's
a number like 192.168.1.165, that is the unique address
that nobody else on your network has.
This is how other devices will address or send information
to your computer.
The second important configuration setting
is the subnet mask.
This is a mask that commonly looks like 255.255.255.0,
or some other combination of numbers
that makes up the subnet mask.
This is a number that is used in conjunction with your IP
address so that your device understands what IP
subnet it happens to live on.
This is important when you're trying
to route outside of your local IP subnet,
and you need to send information to a default gateway.
If your computer doesn't know what subnet it's on,
then it doesn't know what information should
be communicated locally and what information should be
sent to your default gateway.
So we make sure we configure our subnet mask
so that your device knows exactly where this data should
be sent.
And the third configuration setting
that we need to put into our computer
is that default gateway address.
In this example, the default gateway is 192.168.1.1.
This is an IP address that is on the same local subnet
as your IP address, because this is where you're
going to be sending all of the traffic that needs to go
outside of this local subnet.
If you're configuring these settings on a computer,
and you ask someone for the local gateway,
they're going to provide you with the default gateway IP
address, which is another device that happens
to be on your local IP subnet.
You will also very commonly configure
an additional setting, that's not listed here.
That's for your domain name system server, or DNS server.
We'll discuss DNS configuration settings in a future video.
There are other IP addresses configured
on your system that are sometimes
used in special circumstances.
One of these is the loopback address.
This, As the name implies, is an address that references itself.
So if you send any traffic to a loopback address,
you're really sending that to your local machine.
The IPv4 address range for a loopback address
is between 127.0.0.1 through 127.255.255.254.
Normally we just use the first IP address associated
with that, so if you wanted to ping your local loopback
address, you would ping 127.0.0.1.
It may seem unusual that you would
have a separate address that is referenced to your machine,
but this is a great way for your local computer
to determine if the IP stack inside of your system
is working properly.
You can perform that ping, and if you get a response back
from your system, then that means that the IP
stack is working properly.
If you ping that loopback address
and you get no response from your local computer,
then there must be some problem with the TCP/IP stack inside
of your operating system.
There's also a bloc of addresses that have been set aside
as reserved addresses.
When we get into talking about classes of addresses,
these are Class E addresses, and they're
in a range from 240.0.0.1 through 254.255.255.254.
These reserved addresses should not
be configured on a local machine,
or on anything on your production network,
and it would be unusual if you would ever
run across a reserved address, because everyone
understands that these addresses should not be used.
And one special address you might find, especially
if you're working with routers or other devices
with multiple IP addresses, is a virtual IP address, or a VIP.
A virtual IP address that is assigned
internal to your system, but it's not
assigned to a physical network adapter,
instead it's being assigned to a logical network
adapter that only exists inside the software of your system.
These virtual IP addresses are commonly
seen with virtual machines, and you might also
see VIPs used inside of routers so that there is always
a static IP address that you can reference
for that particular router configuration.
Let's break apart this IPv4 address
and really see how it's constructed.
This is our Internet Protocol version 4 address.
Sometimes you might hear this address
referred to as an OSI Layer 3 address.
The addresses that we've been looking at previously
are things that you might recognize.
For example, here's an IP address of 192.168.1.131.
But if you were to convert that decimal value into binary,
you would see that the IP address
are four separate groups of eight bits apiece.
And you can see eight bits in the first group,
eight bits in the second, eight bits and the third,
and lastly, eight bits in that fourth group.
You might also see this referred to as a byte, or an octet.
They are all exactly the same thing.
If you put four of those groups together,
then you have 32 bits, or 4 bytes,
and that is what we use to define an IPv4 address.
We know from our binary math video
that this combination of 1s and 0s could be all 0s
or it could be all 1s.
And if we set all of those bits to 1
and then convert them back to decimal,
you'll find that the maximum value
in any one of these groups is always going to be 255.
If you're just getting into IP addressing,
you may realize that there's an entire set of addresses
on your computer that you've never
had to manually configure.
If you look at the configuration of the IP addresses
on your computer, you may see that they
have been automatically assigned an IP address, a subnet mask,
a gateway, a DNS server, and NTP server,
and perhaps other settings as well.
We're able to automatically assign these IP addresses
by using DHCP, or Dynamic Host Configuration Protocol.
This is a protocol that takes a system that has not
been configured with an IP address,
communicates over the network using this protocol,
and automatically IP addresses are assigned to a local device.
This obviously saves a lot of time for the network
administrator, because they don't have to go to each device
manually to configure IP addresses.
Instead, a user simply turns on their computer,
it communicates automatically to a DHCP server,
and that device receives an IP address
without any user intervention.
DHCP is a common way of automatically assigning
IP addresses on both home networks
and networks in the enterprise, but there
may be times when a DHCP server is not available.
And you may find that your device has been assigned an IP
address, but it wasn't assigned that IP
address from a DHCP server.
In that case, you were probably assigned
an automatic private IP address, or an APIPA.
This is what we call a link-local address,
Because the IP address that's assigned to your computer
is not able to communicate outside of your local subnet.
That means if you've been assigned an APIPA address,
then you're not able to communicate out
to the internet.
This means that if you've been assigned an APIPA address,
you're not able to communicate to the internet
because routers are not able to forward those IP addresses.
One way to tell if you've been assigned an APIPA address
is to simply look at the IP address.
And if this IP address is between the range
of 169.254.0.1 through 169.254.255.254,
then you've been assigned an APIPA address.
This range is actually slightly smaller than this,
because the standard for APIPA takes
the first and last 256 addresses,
and assigns them as reserved.
So the actual range of IP addresses
that can be assigned to a device is between 169.254.1.0
through 169.254.254.255.
If a DHCP server is not available,
then this APIPA assignment process begins.
Your computer picks a random address
in this particular range, and then sends an ARP
to the network to see if anyone else may be currently using
that IP address.
If there's no response to the ARP, then that address is free,
and it's assigned to that computer.
If you are ever working with a device
and you see that it has been assigned an APIPA address,
then you're probably connected to a network that
doesn't have a DHCP server, or the DHCP server on that network
is no longer functioning.