NETWORK
• Relates to: Network+ | CCNA
So, you've decided it isn't good enough anymore to just know IP addresses are
on the Network Layer of the OSI model, you want to apply that knowledge. This
path will ultimately lead you to subnetting. For now, let's separate the IP
classes.
Remember that every IP addy is 32 bits long and is divided into four (the
quartet) octets arranged in dotted-decimal notation. An octet is 8 bits. This is
one way look at an IP address:
216.77.133.249
Here's another:
11011000.01001101.10000101.11111001
It May not look it, right off the bat, but it's the same number. The first
one is in decimal, the second is in binary. I don't want to stray too far from
this article's topic, though I will, briefly, in just a second. Click here for
another Cramsession article with a fuller explanation.
The IP address represented above in binary above contains four octets; the
eight bits clearly designated (not so easily recognized in decimal). The
position of each bit represents the binary number in powers of 2, like so:
2^7 = 128
2^6 = 64
2^5 = 32
2^4 = 16
2^3 = 8
2^2 = 4
2^1 =
2
2^0 = 1
Placing a 1 in any position turns it "on." And remember, in binary, the only
language a computer knows, on = 1, off = 0.
So, returning to our example above, the first octet in the decimal version of
the IP address, it comes out as:
1 = 128
1 = 64
0
1 = 16
1 = 8
0
0
0
So 128 + 64 + 16 + 8 = 216, which was the first octet (in decimal) in the IP
address.
The highest possible value, in decimal, of each octet is 255:
128 + 64 +
32 + 16 + 8 + 4 + 2 + 1 = 255.
When all the bits are "on" you get 11111111. When all the bits are "off" you
get 00000000. So, a binary IP address of 11111111.11111111.11111111.11111111
will be, in decimal, 255.255.255.255 (a flooded broadcast). On the other hand, a
binary IP address of 00000000.00000000.00000000.00000000 will be, in decimal,
0.0.0.0, which in a routing table represents an unknown network or host, and is
typically used to designate the default gateway of last resort.
In theory, the 32-bit IP addressing scheme supports up to 3,720,314,628
hosts. In reality, it won't go that high. Generally, there will be only as many
IP addresses as may be designated by the three most common classes of IP
addresses, A, B and C. There are two more classes, D & E, but they are used
for multicast and experimental purposes respectively, so we won't get into those
here.
The classes are split based on the number in the first Octet. It breaks down
like this:
Class First decimal value
Just as an aside, you'll notice the Class A IP address ends in 127 but that
value really is not to be used. 127.0.0.1 is a special Class A address used for
internal loopback testing and will generate no network traffic. Ping it sometime
and you'll see what I mean.
Now, those are the decimal values. Get ready for a shock. Spotting IP address
classes is even easier in binary. Each class can be identified by looking at the
high order bits (the digits at the left end of the octet) and figuring out where
the first zero falls, that is where the first "off" bit is. In Class A
addresses, the very first high order bit is always off. In Class B, the second
high order bit is always off, the first is on. In Class C, the third high order
bit is always off, the first two are always on. That breaks down like this:
Class High Order Bit Value
So, when we look at our above example again, in
decimal:
216.77.133.249
and in binary
11011000.01001101.10000101.11111001
In decimal, we see the value in the first octet is 216, which means it's a
Class C IP Address. In binary, we look at the high order bits in the first octet
and see that the first high order bit to be off (or 0) is the third one, so this
is a Class C address.
Knowing the class is important because that will determine your default
subnet. It also will indicate the network and host portions of each address. The
default subnet mask for a Class A IP address is 255.0.0.0; Class B is
255.255.0.0 and Class C is 255.255.255.0. By coincidence, (yeah, you bet!) the
network portion of a Class A IP address is the first octet. In Class B, it's the
first two octets. In class C, it's the first three octets. In all three cases,
the host number follows the network portion of the address.
The network portion of any IP address uniquely identifies the network to
which the address belongs. Every workstation, every piece of equipment that can
be assigned an IP address, will share the network portion in its IP address....
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