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IPv4 Classless Subnet equation

  • Last Updated : 09 Aug, 2019

Prerequisite – Classless Addressing, Supernetting
Problem – How to calculate IP address subnet information (Network, Broadcast, First IP, Last IP)?
It’s too simple equation to calculate IPv4 Subnet Network ID.

*Used and Tested with Class C Subnets.

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First Of All, Keep this Subnet Hosts Map in mind (Number of Hosts per Prefix):



Network Prefix:  Number of IPs
24            :      256 IPs
25            :      128 IPs
26            :      64 IPs
27            :      32 IPs
28            :      16 IPs
29            :      8 IPs
30            :      4 IPs 

Using Equation:

Network ID: floor(Host Address/Subnet Number of Hosts) * Subnet Number of Hosts
Broadcast ID: (Host ID + (Subnet Number of Hosts-1))
First Host: Network ID + 1
Last Host: Broadcast ID - 1

Ex1: 192.168.1.65/28:

65/16 = 4.0625
Network ID: 4*16 = 64           (192.168.1.64)
Broadcast ID: 64+(16-1) = 79    (192.168.1.79)
First Host ID: 64 + 1 = 65      (192.168.1.65)
Last Host ID: 79 - 1 = 78       (192.168.1.78)

Ex2: 192.168.20.166/25:

166/128 = 1.296875
Network ID: 1*128 = 128         (192.168.20.128)
Broadcast ID: 128+(128-1) = 255 (192.168.20.255)
First Host ID: 128 + 1 = 129    (192.168.20.129)
Last Host ID: 255 - 1 = 254     (192.168.20.254)

Ex3: 192.168.30.14/29:

14/8 = 1.75
Network ID: 1*8 = 8             (192.168.30.8)
Broadcast ID: 8+(8-1) = 15      (192.168.30.15)
First Host ID: 8 + 1 = 9        (192.168.30.9)
Last Host ID: 15 - 1 = 14       (192.168.30.14)

Ex4: 192.168.20.86/30:

86/4 = 21.5
Network ID: 21*4 = 84           (192.168.20.84)
Broadcast ID: 84+(4-1) = 87     (192.168.20.87)
First Host ID: 84 + 1 = 85      (192.168.20.85)
Last Host ID: 87 - 1 = 86       (192.168.20.86)
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