To understand classful subnetting, first we need to understand how a subnet is constructed. To calculate the number of hosts a network can support, you need to look at the subnet mask. If the subnet mask is 255.0.0.0 for example, it means that the first 8 bits are the Network bits and rest of the 24 bits are host bits. Similarly, if the subnet mask is 255.255.255.0, it means that first 24 bits are network bits and the rest 8 bits are host bits.
Now, in order to construct a subnet, we need to be familiar with a few things.
Network Address: It is the first IP address of a subnet. To determine the network address, set all the host bits to 0. For example, if the IP address is 192.168.1.10, its a class C address and the subnet mask will be 255.255.255.0. Thus, the network address is 192.168.1.0.
First usable host address: It is one number higher than the network address. In our case, it will be 192.168.1.1.
Network broadcast address: This is the last IP address of the subnet. To calculate it, set all the host bits to 1. In our case, if you turn all host bits to 1, it becomes equivalent to 255 in decimal. Thus, it will be 192.168.1.255.
Last usable host address: One number lower than the broadcast address. In our case, it will be 192.168.1.254.
A Practical Example
Challenge: Calculate the Network address, First usable host address, Network broadcast address and Last usable host address from the following IP address:
IP Address: 10.55.211.10
Class A Address
Subnet mask: 255.0.0.0
What is Subnet Mask?
I am assuming that most of you already know what a Subnet mask is, but if you don’t here’s what it is.
It’s a representation of an IP address that tells you about the Network bits and Host bits.
/24 is the CIDR (Classless Inter Domain Routing) notation. All of the 1’s represent the network portion of the subnet mask and 0’s represent the host portion.
Here’s another example. Let’s say we want to calculate the subnet mask from this Binary value:
11111111.11111111.11100000.00000000
255.255.240.0 (Subnet mask) (Convert binary to decimal)
It is a /19 Subnet mask. (Count the number of 1’s)
Related Guides
Subnetting Tutorial Part 1: Binary Math
Subnetting Tutorial Part 3: Classless Subnetting
