How to Calculate Host Bits in CIDR Subnet Calculation

In CIDR (Classless Inter-Domain Routing), an IP address consists of 32 bits (for IPv4), divided into:

• Network bits (define the network)

• Host bits (define devices within the network)

Formula to Calculate Host Bits

Host Bits=32−CIDR Prefix\text{Host Bits} = 32 - \text{CIDR Prefix}

Where CIDR Prefix (e.g., /24) represents the number of network bits.

Step-by-Step Example

1. Find the Host Bits

Let’s say we have a CIDR /26:

Host Bits=32−26=6\text{Host Bits} = 32 - 26 = 6

2. Calculate Total Hosts

Total Hosts=2Host Bits\text{Total Hosts} = 2^{\text{Host Bits}}

For /26:

26=64(Total IPs in subnet)2^6 = 64 \quad (\text{Total IPs in subnet})

3. Find Usable Hosts

Since two addresses are reserved (network ID and broadcast address), the number of usable hosts is:

Usable Hosts=2Host Bits−2\text{Usable Hosts} = 2^{\text{Host Bits}} - 2

For /26:

64−2=62(Usable Hosts)64 - 2 = 62 \quad (\text{Usable Hosts})

Quick Reference Table

CIDR	Network Bits	Host Bits	Total IPs	Usable Hosts
/30	30	2	4	2
/29	29	3	8	6
/28	28	4	16	14
/27	27	5	32	30
/26	26	6	64	62
/25	25	7	128	126
/24	24	8	256	254
/16	16	16	65,536	65,534

How to Choose CIDR for a Required Number of Hosts

If you need X hosts, find the smallest CIDR where:

2Host Bits−2≥X2^{\text{Host Bits}} - 2 \geq X

🔹 Example: Need 500 hostsFind smallest CIDR where:

2n−2≥5002^n - 2 \geq 500

• 2^8 = 256 - 2 = 254 (Too small)

• 2^9 = 512 - 2 = 510 (Enough!)

✅ CIDR /23 supports 510 usable hosts.