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.
CONVENTIONAL CLASSFULL ADDRESSING
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).
|First Octet of|
|Lowest Value of First Octet (Binary)||Highest Value of First Octet (Binary)||Range of First Octet Values (Decimal)||Theoretical|
IP Address Range
|Class A||0 to 127||0.0.0.0 to|
|Class B||128 to 191||126.96.36.199 to|
|Class C||192 to 223||192.0.0.0 to|
|Class D||224 to 239||188.8.131.52 to|
|Class E||240 to 255||240.0.0.0 to|
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.
Network ID Bits
|Number of Usable|
Network ID Bits
|Number of Possible|
|Number of Host ID|
Per Network ID
|Class A||8 - 1 = 7||27 - 2 = 126||224 - 2 = 16,777,214|
|Class B||16 - 2 = 14||214 = 16,384||216 - 2 = 65,534|
|Class C||24 - 3 = 21||221 = 2,097,152||28 - 2 = 254|
|Class D||Reserved for IP multicasting|
|Class E||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 ID||Host ID||Class A|
|Special Meaning and Description|
|Network ID||Host ID||184.108.40.206||220.127.116.11||18.104.22.168||Refers to a specific device.|
|Network ID||All Zeros||22.214.171.124||126.96.36.199||188.8.131.52||Refers to a Specified Network : With a 0 at the end of the address, refers to an entire network.|
|All Zeros||Host ID||0.16.208.24||0.0.78.145||0.0.0.28||Specified 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 Zeros||All Zeros||0.0.0.0||Refers 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 ID||All Ones||184.108.40.206||220.127.116.11||18.104.22.168||All Hosts on the Specified Network : Used for broadcasting to all hosts on the local network.|
|All Ones||All Ones||255.255.255.255||All 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.|
|Equivalent Classful Address||Equivalent|
|0.0.0.0||0.255.255.255||Class A network 0.x.x.x||0/8||Reserved|
|10.0.0.0||10.255.255.255||Class A network 10.x.x.x||10/8||Class A private address block|
|127.0.0.0||127.255.255.255||Class A network 127.x.x.x||127/8||Loopback address block|
|22.214.171.124||126.96.36.199||Class B network 128.0.x.x||128.0/16||Reserved|
|169.254.0.0||169.254.255.255||Class B network 169.254.x.x||169.254/16||Class B private address block reserved for automatic private address allocation|
|172.16.0.0||172.31.255.255||16 contiguous Class B networks from 172.16.x.x through 172.31.x.x||172.16/12||Class B private address blocks|
|188.8.131.52||184.108.40.206||Class B network 191.255.x.x||191.255/16||Reserved|
|192.0.0.0||220.127.116.11||Class C network 192.0.0.x||192.0.0/24||Reserved|
|192.168.0.0||192.168.255.255||256 contiguous Class C networks from 192.168.0.x through 192.168.255.x||192.168/16||Class C private address blocks|
|18.104.22.168||22.214.171.124||Class C network 223.255.255.x||223.255.255/24||Reserved|
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