- [Instructor] One of the most important topics
in any discussion of TCPIP is IP addressing.
An IP address is a numeric identifier
assigned to each machine on an IP network.
It designates the specific location
of a device on the network.
An IP address is a software or logical address,
not a hardware address like our Mac addresses.
IP addressing was designed to allow hosts on one network
to communicate with hosts on a different network,
regardless of the type of LAN
the hosts are participating in.
Shown in the graphic, an IP address
is comprised of 32 bits of information.
These bits are divided into four bytes,
or four sections.
They're referred to as octets or bytes,
each containing eight bits.
The 32 bit IP address is structured, or hierarchical,
as opposed to a flat or non-hierarchical address.
Although either type of addressing scheme
could've been used, hierarchical addressing
was chosen for a good reason.
The advantage of the scheme is that it can handle
a large number of addresses, namely 4.3 billion.
The network address, which is defined
in either the first byte or bytes of the IP address,
uniquely identifies each network.
Every machine on that network shares the same
network address as part of its IP address.
Subdividing an IP address into a network and node address
is determined by the class designation of one's network.
Taking a look at this graphic,
we can see that there's five classes of addresses,
Class A through Class E.
Class A, B, and C are used to address
our networks and hosts.
Class D and E are reserved, Class D for multi-cast use,
and Class E is wasted, or research,
and we just don't use it.
The first byte in a Class A network defines the network,
which gives us 24 bits for defining hosts.
This gives us the most flexibility.
Class A is mostly used in our enterprises today.
Class B, the first two bytes represent the network,
and the last 16 bits represent the hosts on each network.
In a Class C, the first three bytes represent the network,
and the last byte, or only eight measly bits,
represent the hosts on each network.
Now, the designers of the IP address scheme
decided that the first bit of the first byte
in a Class A network address
must always be off, or zero.
If I have a Class A address,
the first bit of the first byte is off.
I take the other seven bits and turn them on,
and I get my Class A range in the first octet,
zero to 127.
However, Cisco reserved zero in the Class A,
and 127 is used for diagnostics or loopback.
So this gives us a range of one to 126.
In a Class B range, the first bits must be 10.
This defines our first two bits in the first byte.
We turn all the other bits off and on,
and this gives us our Class B range.
So by looking at an IP address,
I'll know whether it's Class A or B at this point,
because Class A is from one to 126,
Class B is from 128 to 191,
and Class C, the first three bits
must be one, one, zero,
so we turn off the other bits, turn them all on,
which gives us our range of 192 to 223.
By looking at an IP address,
I can tell whether it's Class A, B, or C.
So, for example, if I had this address
of 10.0.0.0, I would know that that fits nicely
in the Class A range.
There's one more consideration
when talking about IP addresses,
and that is something called RFC 1918.
And this gives us our private address ranges.
Private address ranges cannot be
routed through the Internet.
Private IP addresses are schemed
to conserve public addresses.
For Class A, our range is
anything that starts with 10, really.
10.0.0.0 to 10.255.255.255.
For Class B, the reserved range is 172.16
to 172.31, and that gives us
really large range in the Class B.
In Class C, it's just anything that starts with 192.168.
It's very common to see these addresses
when you're at work, or even at home,
if you were to go out and buy a Linksys or Netgear
or something router and connect it up,
they're gonna use the 192.168.
Typically, though, enterprises,
we're gonna use the 10 network,
because that gives us the most flexibility.