IPv4 Address Classes and Subnet Masks

Overview

IPv4 addresses are divided into different classes to organize and allocate IP address ranges efficiently. Each class has a specific range of first octet addresses and a corresponding default subnet mask.

The network portion of an IP address is the network ID that uniquely identify the network

Class Details

Class A

  • First Octet Range: 1 - 126

  • Default Subnet Mask: 255.0.0.0 (/8 ⇒ 11111111.0.0.0)

  • Format: NETWORK.HOST.HOST.HOST

  • Available Host Addresses: Up to 16 million hosts per network

  • Suitable for: Very large networks, such as the one used by Internet Providers

Class B

  • First Octet Range: 128 - 191

  • Default Subnet Mask: 255.255.0.0 (/16 ⇒ 11111111.11111111.0.0)

  • Format: NETWORK.NETWORK.HOST.HOST

  • Available Host Addresses: Up to 65.000 hosts per network

  • Suitable for: Medium to large networks for medium to large organisations

Class C

  • First Octet Range: 192 - 223

  • Default Subnet Mask: 255.255.255.0 (/24 ⇒ 11111111.11111111.11111111.0)

  • Format: NETWORK.NETWORK.NETWORK.HOST

  • Traditional Network Available Host Addresses: Up to 254 hosts (2 reserved address: network and broadcast)

  • AWS Available Host Addresses: Up to 251 hosts per network (AWS reserves 5 addresses: network, VPC router, DNS, future use, broadcast)

  • Suitable for: Small networks for small organisations and homes.

Binary Structure Example

Using Class A as an example:

Subnet Mask: 255.0.0.0
Binary:      11111111.00000000.00000000.00000000
             NETWORK  .   HOST   .   HOST   .   HOST

Important Notes

  • The range 127 is reserved for loopback testing and is not included in any class

  • These classes represent the traditional classful networking scheme

  • Modern networks typically use Classless Inter-Domain Routing (CIDR) for more flexible subnet allocation

  • For Traditional network the number of possible hosts is calculated using the formula 2^n - 2, where n is the number of host bits

  • For AWS network the number of possible hosts is calculated using the formula 2^n - 5, where n is the number of host bits.

Host Capacity

The structure can produce up to 16 million hosts per network, making it particularly suitable for large organizations and networks requiring extensive host addressing capabilities.

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