IPv4 Classless Subnet equation (original) (raw)

Last Updated : 11 Jul, 2025

Prerequisite - Classless Addressing, Supernetting

Problem - How to calculate Network Address, Broadcast Address, First and Last IP address of a given IP Address in the CIDR(Classless Inter-Domain Routing) Notation.

There is a simple equation to calculate the above mentioned Addresses which is used and tested with Class C Networks_._

Firstly, remember the No. of IP Addresses with respect to the Network Prefix(the value after the Slash(/) in CIDR Notation) as shown in the Table below.

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

Note that in the above table the Number of Hosts for mentioned Network Prefixes will be 2 less than the No. of IP Addresses, because the First IP Address is the Network Address and the Last IP Address is the Broadcast Address.

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

Ex2: 192.168.20.166/25:

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

Ex3: 192.168.30.14/29:

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

Ex4: 192.168.20.86/30:

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

The above Examples work for only Class C Networks, as we are assuming the host part of the IP Address to be the last octet as in the case of Class C Networks.