Every IT should Know: IP Address Classes

Here is info on IP Classes and some methods to memorize them.

There are 5 Classes of IP addresses here are the ranges and default subnet mask:

Class A - 1 through 127    Subnet: 255.0.0.0

Class B - 128 though 191  Subnet: 255.255.0.0

Class C - 192 through 223 Subnet: 255.255.255.0

Class D - 224 through 239 (Reserved for multicast)

Class E - 240 through 254 (Reserved for experimental and research use)

(Class A 127.0.0.0 to 127.255.255.255 cannot be used as they are reserved for diagnostics and testing)

Leading bits:

To understand leading bits and IP Classes you need to know binary.  

To simplify binary imagine 4 people lined up from right to left number them starting with 1 number doubling the number each person over  

Now cover some of the numbers and leave others uncovered

Now add up the uncovered number.   That is how binary works each person is representing a bit the bit is either on represented by 1 or off represented by 0 then you just add up the bits that are on.

An IP addresses is 4 octets comprised of 8 bits  instead of just going to 8 now you have bits representing as much as 128.   In binary 10000000 is 128 because the the first bit represents 128 and it is the only one on.

Now that you have a basic idea of binary you can understand the logic behind  IP address class ranges and how they are set.   Each IP address Class has leading bits these leading bits can not change.  To better understand look at the chart of IP Classes the leading bits have been highlighted.

Class A leading Bits 0
 0.  0.  0.  0 = 00000000.00000000.00000000.00000000
127.255.255.255 = 01111111.11111111.11111111.11111111
(Notice that the leading bit never ends right before the bit of 128 and that is the next Class of IP)

            
Class B leading Bits 10
128.  0.  0.  0 = 10000000.00000000.00000000.00000000
191.255.255.255 = 10111111.11111111.11111111.11111111

                 
Class C leading Bits 110
192.  0.  0.  0 = 11000000.00000000.00000000.00000000
223.255.255.255 = 11011111.11111111.11111111.11111111
                
Class D leading Bits 1110
224.  0.  0.  0 = 11100000.00000000.00000000.00000000
239.255.255.255 = 11101111.11111111.11111111.11111111
              
Class E leading Bits 1111
240.  0.  0.  0 = 11110000.00000000.00000000.00000000
255.255.255.255 = 11111111.11111111.11111111.11111111
                          

Available Networks and Hosts IP's

The Default Subnet marks what portion of he IP address is Network with 255 and what is Host with 0.

Look at the chart below the correlating number for available Networks and Hosts.

Class	First octet Range	Network portion=N Host portion=H	Available Networks	Avalable Hosts per Network
A	1–127	N.H.H.H 255.0.0.0	126 (127 is reserved)	16,777,214
B	128–191	N.N.H.H 255.255.0.0	16,384	65,534
C	192–223	N.N.N.H 255.255.255.0	2,097,152	254

(The reason class C is not 255 available host per network is that you have 1 host for broadcast)

How to remember Private IP Addresses

Class A- 10.0.0.0 to 10.255.255.255
All Class A IP's that start with 10 are private

Class B 172.16.0.0 to 172.31.255.255
There are 16 private networks available in class B so the 2nd octet is 16 then including  16 count up 16 times  16, 17,18,19........... 31 that is the 2nd octet of the highest Class B private IP remaining octets go from 0-255.

Class C- 192.168.0.0 to 192.168.255.255
What is the default IP on your home router?