An Internet Protocol address (IP address) is a layer 3 logical address assigned by a network administrator. IP addresses are used to identify specific devices on a network.

An IP address is a 32-bit binary address usually written in dotted decimal formats. These 32 bits are further subdivided into four 8 bit segments called octets. Each octet is separated by a period.

Also Read: OSI Model 7 Layers Explained

Every IP address can be broken down into two main portions.

  1. Network Address Portion (Network ID)
  2. Host Address Portion (Host ID)

Network address portion is used to identify a specific network. Routers maintain routing tables that contain the network addresses. Host address portion is used to identify a specific endpoint on a network such as servers, printers, computers, mobile phones, etc… (Please Note – Routers build their routing tables based on network address not based on Host addresses.)

IP address Structure
IP address Structure

Address Classes/Classful Networks

This is a network addressing architecture used in the Internet from 1981 until the introduction of CIDR (Classless Inter-Domain Routing) in 1993.

This method divides IPv4 address space into five classes.

  1. Class A
  2. Class B
  3. Class C
  4. Class D
  5. Class E

Class A, Class B and Class C  provide unicast addresses. Class D is for multicast networking and the class E address range is reserved for future or experimental purposes.

These Address classes were determined and allocated by the Internet Assigned Numbers Authority (IANA)

Class A

In Class A addresses first bit of the first octet is always set to Zero.

00000000 – 01111111

Therefore in Class A addresses the first octet ranges from 0 – 127.

0.0.0.0 – 127.255.255.255

But 127 is reserved for loopback IP addresses. As an example, you can’t configure an IP address of 127.0.0.1 as a static IP on a PC. 0 is reserved for default network. So that can’t be used either to configure an IP address on a PC. As an example, you can’t configure an IP address of 0.1.1.1 as a static IP on a PC.

Because of the above-mentioned reasons the actual range of Class A addresses is from 1 – 126.

1.0.0.0 – 126.255.255.255

In a Class A Address the first 8 Bits denotes the Network Portion and the last 24 Bits denotes the Host Portion.

Class A Address Structure
Class A IP Address Structure

Class B

In Class B addresses first two bit of the first octet is always set to 10. (One and Zero).

10000000 – 10111111

Therefore in Class B addresses the first octet ranges from 128 – 191.

128.0.0.0 – 191.255.255.255

In a Class B Address the first 16 Bits denotes the Network Portion and the last 16 Bits denotes the Host Portion.

Class B IP Address Structure
Class B IP Address Structure

Class C

In the first octet of Class C addresses first 3 Bits are set to 110. (One One Zero)

11000000 – 11011111

Therefore in Class C addresses the first octet ranges from 192 – 223.

192.0.0.0 – 223.255.255.255

In a Class C Address the first 24 Bits denotes the Network Portion and the last 8 Bits denotes the Host Portion.

Class C IP Address Structure
Class C IP Address Structure

Class D

In Class D addresses the very first four bits of the first octet are set to 1110. (One One One and Zero)

11100000 – 11101111

Therefore in Class D addresses the first octet ranges from 224 – 239.

224.0.0.0 – 239.255.255.255

These addresses are reserved for Multi casting. In multi casting data is not transmit for a particular host. Therefore it’s not necessary to extract host address from the IP address.

Class E

Class E is reserved for experimental purposes.

In Class E addresses the very first four bits of the first octet are set to 1111. (One One One and One)

11110000 – 11111110

Therefore in Class E addresses the first octet ranges from 240 – 254.

240.0.0.0 – 254.255.255.254

Summary



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