How do you determine the size of a subnet?

The size of a subnet is the number of IP addresses (or hosts) that belong to one subnet. It is determined by the subnet mask (CIDR prefix).

Short and simple: the size of a subnet depends on how many bits in the IP address are allocated to hosts.

---

#The basic principle

For IPv4:

• total in the address: 32 bits
• part of them: the network
• the rest: the hosts

Formula:

The - 2 is because:

• one address is the network address
• one is the broadcast address

---

#How to determine subnet size step by step

#Step 1. Look at the prefix (/24, /26, and so on)

Example:

192.168.1.0/24

• 32 - 24 = 8 bits for hosts

---

#Step 2. Calculate the number of addresses

2^8 = 256 addresses

---

#Step 3. Calculate the number of hosts

256 - 2 = 254 hosts

---

#Examples of the most popular subnets

---

#A quick "at a glance" method (useful for an interview)

If the mask ends in:

• .0 → /24 → 254 hosts
• .128 → /25 → 126 hosts
• .192 → /26 → 62 hosts
• .224 → /27 → 30 hosts
• .240 → /28 → 14 hosts

---

#What is important to remember

• the size of a subnet is not the number of devices, but the maximum possible number
• the network and broadcast addresses are always reserved
• routers operate on subnets, not on individual IPs