# IP question?

#### mikeshn

##### Senior member
Hello

I really don't understand the necessity to divide IP's into 3 different classes. Can someone give a good reason?

Thanks

#### Fatt

##### Senior member
You have to look at in in binary for it to make sense.

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A "class A" address always has the 1st bit turned off, or set to 0:

0xxxxxxx
0000000 - 01111111 = 0 - 127
_________________________________________

A "class B" address always has the 1st bit turned on and the 2nd bit turned off:

10xxxxxx
10000000 - 10000000 = 128 - 191
_________________________________________

A "class C" address always has the first two bits turned on and the 3rd bit off:

110xxxxx
11000000 - 11011111 = 192 - 223
_________________________________________

A "class D" address always has the first three bits turned on and the 4th off and a "Class E" address has the 1st 4 bits turned on.

11100000 - 11101111 = 224 - 239 = class D = Multicast addresses

11110000 - 11111111 = 240 - 255 = class E = Scientific addresses
__________________________________________________________________

There are also some special purpose IP addresses that are "non-routable.

Such as private networks (10.10.10.0 & 192.168.0.0), the loopback address (127.0.0.1) and and address that is all Ones or all Zeros.
0.0.0.0 is interpreted to mean "this network or segment"
255.255.255.255 is interpreted to mean "all networks or segments"

The last part of my post is a little oversimplified but I'm getting bored so your eyes must really be glazing over.
The important part is for the 3 basic classes. (A, B & C)

#### Fatt

##### Senior member
Oooops...typo
_________________________________________

A "class B" address always has the 1st bit turned on and the 2nd bit turned off:

10xxxxxx
10000000 - 10000000 = 128 - 191
_________________________________________

That should actually be:

10000000 - 10111111 = 128 - 191

#### mikeshn

##### Senior member
thanks for fabiluos explanation ...
But why we need class A or Class B or Class C ..

I't simple to have just IP..

Also

I don't undersand the a+ x(x-ecexute, what is a?) .

thanks

#### Fatt

##### Senior member
It has to do with how many network bits you have versus how many host bits, as well as how many you can borrow for subnetting.
It also decreases overhead since a router only needs to know the 1st 3 bits of a 32 bit IP address to know the class although I must admit that I don't know what the practical advantage of that.

and uh...

chmod a+x /bin/laden is a unix command. It gives everyone permission to execute /bin/laden.

thanks a lot
I got it

#### Woodie

##### Platinum Member
The practical advantage is subnetting, which then allows (better) routing. Router don't (shouldn't) have to know about individual IP addresses, just the subnets that are connected to it.