How can we identify the class of the IP address? Whether it is class A, class B or class C?

Looking at the first octet of an IP address, you can identify the class of that address: Class A =1-126 Class B =128 -191 Class C =192 -223 Class D =224 –239 Class E =240 –254 Looking at the first octet to determine the class:64.192.29.2 = Class A =64.0.0.0   172.16.32.4 = Clsss B =172.16.0.0   208.16.14.1 = Class C =208.16.14.0

The Internet community originally defined five address classes to accommodate networks of varying sizes.
Microsoft TCP/IP supports class A, B, and C addresses assigned to hosts.
The class of address defines which bits are used for the network ID and which bits are used for the host ID.
It also defines the possible number of networks and the number of hosts per network.
Class A Class A addresses are assigned to networks with a very large number of hosts.
The high-order bit in a class A address is always set to zero.
The next seven bits (completing the first octet) complete the network ID.
The remaining24 bits (the last three octets) represent the host ID.
This allows for126 networks and16,777,214 hosts per network.
Figure1.4 illustrates the structure of class A addresses.
Figure1.4 Class A IP Addresses   Class B Class B addresses are assigned to medium-sized to large-sized networks.
The two high-order bits in a class B address are always set to binary10.
The next14 bits (completing the first two octets) complete the network ID.
The remaining16 bits (last two octets) represent the host ID.
This allows for16,384 networks and65,534 hosts per network.
Figure1.5 illustrates the structure of class B addresses.
Figure1.5 Class B IP Addresses   Class C Class C addresses are used for small networks.
The three high-order bits in a class C address are always set to binary110.
The next21 bits (completing the first three octets) complete the network ID.
The remaining8 bits (last octet) represent the host ID.
This allows for2,097,152 networks and254 hosts per network.
Figure1.6 illustrates the structure of class C addresses.
Figure1.6 Class C IP Addresses   Class D Class D addresses are reserved for IP multicast addresses.
The four high-order bits in a class D address are always set to binary1110.
The remaining bits are for the address that interested hosts recognize.
Microsoft supports class D addresses for applications to multicast data to multicast-capable hosts on an internetwork.
  Class E Class E is an experimental address that is reserved for future use.
The high-order bits in a class E address are set to1111.
Table1.11 is a summary of address classes A, B, and C that can be used for host IP addresses.
  Table1.12 Class Ranges of Network IDs Address Class First Network ID Last Network ID Class A 1.0.0.0 126.0.0.0 Class B 128.0.0.0 191.255.0.0 Class C 192.0.0.0 223.255.255.0

Interesting answers by others but difficult to understand as words.

Try this link instead.

http://www.hill2dot0.com/wiki/images/4/40/IP_Address_Classes.jpg

                                               Start address   End address Class A    0                   0.0.0.0             127.255.255.255 Class B    10                128.0.0.0          191.255.255.255 Class C    110              192.0.0.0          223.255.255.255  

below is ful details of ip Address    ClassLeadingbitsSize of networknumber bit fieldSize of restbit fieldNumberof networksAddressesper networkStart addressEnd addressA 0 8 24 128 (27) 16,777,216 (224) 0.0.0.0 127.255.255.255 B 10 16 16 16,384 (214) 65,536 (216) 128.0.0.0 191.255.255.255 C 110 24 8 2,097,152 (221) 256 (28) 192.0.0.0 223.255.255.255 and also we have two more class C & D  also avialable but that we use for Expermental Purpose  only but calss A, B & C are the most use IP Address in Comany or Organization for LAN Use 

by looking to the first octet :Class  A :1 to126Class  B :128 to191Class  C :192 to223

it's very easy  look at the1st octet 

if it's class A the rang it's between0 to126

if it's class B the rang btween128 to191

it it's class C the rang btween192-223

looking the first octet of an IP address for excample class A addresses started from 1 to126 class B addresses started from 128 to191 Class C addresses started from 192 to223 Class D addresses started from 224 to239 Class E adddresses started from 240 to255    

Class A Network

In a Class A Network binary address start with0, therefore the decimal number can be anywhere from1 to126. The first8 bits (the first octet) identify the network and the remaining24 bits indicate the host within the network. An example of a Class A IP address is102.168.212.226, where "102" identifies the network and "168.212.226" identifies the host on that network.

Class B Network

In a Class B Network, binary addresses start with10, therefore the decimal number can be anywhere from128 to191. The number127 is reserved for loopback and is used for internal testing on the local machine. The first16 bits (the first two octets) identify the network and the remaining16 bits indicate the host within the network. An example of a Class B IP address is168.212.226.204 where "168.212" identifies the network and "226.204" identifies the host on that network.

Class C Network

Binary addresses start with110, therefore the decimal number can be anywhere from192 to223. The first24 bits (the first three octets) identify the network and the remaining8 bits indicate the host within the network. An example of a Class C IP address is200.168.212.226 where "200.168.212" identifies the network and "226" identifies the host on that network.

Class A

إذا رأينا أي عنوان IP يبتدئ بأي رقم بين0 و127 سنعرف أنه ينتمي الى المدى Class A

 

Class B 

عناوين هذا المدى تتميز بما قرره المصممين من أن أول bit من أول byte من عنوان الشبكة المنتمي للمدى Class B لابد أن تكون قيمته1

 

Class C

عناوين هذا المدى تتميز بما قرره المصممين من أن البت الأول و الثاني من البايت الأول يحملان القيمة1 بينما يحمل البت الثالث القيمة0 دوما