IP addresses are 32-bit binary numbers, which can be expressed in binary, hexadecimal, or decimal form. Most commonly, they are expressed by dividing the 32 bits into four bytes and converting each to decimal, then separating these numbers with dots to create dotted decimal notation.

Since IP addresses are 32 bits long, the total address space of IPv4 is 232 or 4,294,967,296 addresses. However, not all of these addresses can be used, for a variety of reasons.

Every IP address has two parts, a network part and a host part

• Network identity (network ID) is a portion of the IP address that is used to identify individuals or devices on a network such as a local area network or the Internet.

• In network addressing, the host address, or the host ID portion of an IP address, is the portion of the address used to identify hosts (any device requiring a Network Interface Card, such as a PC or networked printer) on the network.

The classful IP addressing scheme divides the IP address space into five classes, A through E, of differing sizes. Classes A, B, and C are the most important ones, designated for conventional unicast addresses. Class D is reserved for IP multicasting, and Class E is reserved for experimental use.

 Step 1 − If the first bit is 0, then it is a Class A address.If it is 1, continue to step 2. Step 2 − If the Second bit is 0, then it is a Class B address.If it is 1, continue to step 3. Step 3 − If the third bit is 0, then it is a Class C address.If it is 1, continue to step 4. Step 4 − If the fourth bit is 0, then it is a Class D address.If it is 1, it is a Class E address.

We generally work with addresses in dotted decimal notation, not in binary, but the address range of each class can be determine from binary notation.

For example - In Class B the first two bits of the first octet are 10. The remaining bits can be any combination of ones and zeros. This is normally represented as 10xx xxxx (shown as two groups of four for readability). Thus, the binary for the first octet can range from 1000 0000 to 1011 1111 (128 to 191 in decimal).

Class
First Octet of
Lowest Value of First Octet (Binary)Highest Value of First Octet (Binary)Range of First Octet Values (Decimal)Theoretical
Class A
0xxx xxxx
0000 0000
0111 1111
0 to 1270.0.0.0 to
127.255.255.255
Class B
10xx xxxx
1000 0000
1011 1111
128 to 191128.0.0.0 to
191.255.255.255
Class C
110x xxxx
1100 0000
1101 1111
192 to 223192.0.0.0 to
223.255.255.255
Class D
1110 xxxx
1110 0000
1110 1111
224 to 239224.0.0.0 to
239.255.255.255
Class E
1111 xxxx
1111 0000
1111 1111
240 to 255240.0.0.0 to
255.255.255.255

Certain IP addresses cannot be used because they have special meaning. For example, 255.255.255.255 is a reserved broadcast address.

The range for Class A is from 0 to 127, but the networks 0 and 127 are reserved; 127 is the network that contains the IP loopback address. The address is used to establish an IP connection to the same machine or computer being used by the end-user. The most commonly use IP address is 127.0.0.1, also referred to as the "localhost"; however, using any IP address in the range of 127. * . * . * will function in the same or similar manner.

Establishing a network connection to the 127.0.0.1 loopback address is accomplished in the same manner as establishing one with any remote computer or device on the network. The primary difference is that the connection avoids using the local network interface hardware. System administrators and application developers commonly use 127.0.0.1 to test applications. When establishing an IPv4 connection with 127.0.0.1 will normally be assigned subnet mask 255.0.0.1.

As you’ve seen, the classes A, B and C are differ in the number of bits (and octets) used for the network ID compared to the host ID. The number of different networks possible in each class is a function of the number of bits assigned to the network ID, and likewise, the number of hosts possible in each network depends on the number of bits provided for the host ID.

Based on this information, you can calculate the number of networks in each class, and for each class, the number of host IDs per network.

Class
Number of
Network ID
Bits
Number of
Host ID
Bits
Number of
Network ID Bits
Used To
Identify Class
Number of Usable
Network ID Bits
Number of Possible
Network IDs
Number of Host ID
Per Network ID
Class A
8
24
1
8 - 1 = 727 - 2 = 126224 - 2 = 16,777,214
Class B
16
16
2
16 - 2 = 14214 = 16,384216 - 2 = 65,534
Class C
24
8
3
24 - 3 = 21221 = 2,097,15228 - 2 = 254
Class D
n/a
n/a
Reserved for IP multicasting
Class E
n/a
n/a
Reserved for experimental use

For each network ID, two host IDs cannot be used : the host ID with all zeros and the ID with all ones. These are addresses with special meanings, as described in the table given below.

Network IDHost IDClass A
Example
Class B
Example
Class C
Example
Special Meaning and Description
Network IDHost ID95.16.208.24152.65.78.145220.8.162.28Refers to a specific device.
Network IDAll Zeros95.0.0.0152.65.0.0220.8.162.0Refers to a Specified Network : With a 0 at the end of the address, refers to an entire network.
All ZerosHost ID0.16.208.240.0.78.1450.0.0.28Specified Host on The Current Network : This addresses refers to a specific host on the current or default network when the network ID is not known or when it doesn’t need to be explicitly stated.
All ZerosAll Zeros0.0.0.0Refers to Itself : Used by a device to refer to itself when it doesn’t know its own IP address. The most common use is when a device attempts to determine its address using a hostconfiguration protocol like DHCP.
Network IDAll Ones95.255.255.255152.65.255.255220.8.162.255All Hosts on the Specified Network : Used for broadcasting to all hosts on the local network.
All OnesAll Ones255.255.255.255All Hosts on the Network : Specifies a global broadcast to all hosts on the directly connected network. Note that there is no address that would imply sending to all hosts everywhere on the global Internet, since this would be very inefficient and costly.

 NOTES - The missing combination from the above table is that of the network ID being all ones and the host ID normal. Semantically, this would refer to “all hosts of a specific ID on all networks,” which doesn’t really mean anything useful in practice, so it’s not used. Note also that, in theory, a special address where the network ID is all zeros and the host ID is all ones would have the same meaning as the all-ones limited broadcast address.

RESERVED, PRIVATE AND LOOPBACK IP ADDRESSES

Range
Range
Description
0.0.0.00.255.255.255Class A network 0.x.x.x0/8Reserved
10.0.0.010.255.255.255Class A network 10.x.x.x10/8Class A private address block
127.0.0.0127.255.255.255Class A network 127.x.x.x127/8Loopback address block
128.0.0.0128.0.255.255Class B network 128.0.x.x128.0/16Reserved
169.254.0.0169.254.255.255Class B network 169.254.x.x169.254/16Class B private address block reserved for automatic private address allocation
172.16.0.0172.31.255.25516 contiguous Class B networks from 172.16.x.x through 172.31.x.x172.16/12Class B private address blocks
191.255.0.0191.255.255.255Class B network 191.255.x.x191.255/16Reserved
192.0.0.0192.0.0.255Class C network 192.0.0.x192.0.0/24Reserved
192.168.0.0192.168.255.255256 contiguous Class C networks from 192.168.0.x through 192.168.255.x192.168/16Class C private address blocks
223.255.255.0223.255.255.255Class C network 223.255.255.x223.255.255/24Reserved

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