CIDR Subnetting Calculator: Hosts, Subnet Mask & CIDR Table 🌎

How It's Calculated

CIDR subnetting is based on two core formulas derived from the 32-bit IPv4 address space:

Host bits      = 32 − prefix_length
Total addresses = 2 ^ host_bits
Usable hosts   = 2 ^ host_bits − 2        ← subtract network & broadcast
Subnet mask    = 0xFFFFFFFF << host_bits   → converted to dotted-decimal
/24 subnets    = 2 ^ (prefix_length − 24)  (only when prefix ≥ 24)
             OR 1 / 2 ^ (24 − prefix_length) expressed as a fraction (prefix < 24)

Example — /26:

Host bits = 32 − 26 = 6

Usable hosts = 2^6 − 2 = 62

Mask in binary: 11111111.11111111.11111111.11000000 → 255.255.255.192

This is 1/4 of a /24 block (4 subnets of /26 fit in one /24)

Example — /22:

Host bits = 32 − 22 = 10

Usable hosts = 2^10 − 2 = 1,022

Mask: 11111111.11111111.11111100.00000000 → 255.255.252.0

Spans 4 contiguous /24 blocks

Example — /30:

Host bits = 2 → Usable hosts = 2

Mask: 255.255.255.252

Classic choice for point-to-point WAN or router interconnects

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CIDR Prefix → Subnet Mask → Hosts Reference Table

This is the single table most engineers keep on a sticky note. Find your prefix on the left; read off the mask and how many devices it holds.

CIDR	Subnet Mask	Usable Hosts	/24 Equivalent	Common Use
/8	255.0.0.0	16,777,214	65,536 /24s	Large ISP / Class A legacy
/16	255.255.0.0	65,534	256 /24s	Enterprise campus / AWS VPC max
/20	255.255.240.0	4,094	16 /24s	Large office, cloud region subnet
/22	255.255.252.0	1,022	4 /24s	University building / data center rack
/23	255.255.254.0	510	2 /24s	Medium office LAN
/24	255.255.255.0	254	1 /24	Standard LAN segment
/25	255.255.255.128	126	½ /24	Split LAN (wired vs. wireless)
/26	255.255.255.192	62	¼ /24	Department VLAN
/27	255.255.255.224	30	⅛ /24	Small team / IoT segment
/28	255.255.255.240	14	1/16 /24	Printer/server cluster
/29	255.255.255.248	6	1/32 /24	Small branch office
/30	255.255.255.252	2	1/64 /24	Point-to-point link
/31	255.255.255.254	2*	1/128 /24	P2P (RFC 3021, no broadcast)
/32	255.255.255.255	1 (host route)	1/256 /24	Loopback / static host route

> /31 special case: RFC 3021 (2000) allows /31 for point-to-point links without a network/broadcast address, giving 2 usable host addresses despite the formula yielding 0.

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Typical Cases

Case 1: Standard Office LAN (/24)

A small company needs one subnet for 180 employees. A /24 provides 254 usable addresses — enough headroom without wasting a full /23 (510 hosts). Subnet mask: 255.255.255.0. Network: 192.168.1.0/24, hosts range 192.168.1.1–192.168.1.254, broadcast 192.168.1.255.

Case 2: AWS VPC Subnet Planning (/26)

An engineer creates a public subnet in us-east-1a. AWS reserves 5 addresses (network, router, DNS, future use ×2, broadcast), so a /26 yields 59 usable IPs (not 62). For 100 EC2 instances, a /25 (123 usable in AWS) is the right pick.

Case 3: ISP Backbone Point-to-Point (/30)

A router-to-router link between two ISP PoPs uses /30 (255.255.255.252): 4 total addresses, 2 usable. IP A = x.x.x.1, IP B = x.x.x.2, broadcast = x.x.x.3. No wasted space, no routing overhead.

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Common Errors

1. Forgetting to subtract 2 from total addresses. Every subnet loses the network address (all host bits = 0) and the broadcast address (all host bits = 1). A /28 has 16 total addresses but only 14 usable hosts.

2. Confusing prefix length with host count. /24 does NOT mean 24 hosts — it means 24 network bits, leaving 8 host bits → 254 hosts. A common beginner error that leads to undersized subnets.

3. Ignoring cloud provider reservations. AWS, Azure, and GCP each reserve 5 addresses per subnet (not 2 like standard IPv4), so always subtract 5 instead of 2 when sizing cloud subnets.

4. Using the wrong mask for VLANs. Entering 255.255.255.0 when you need 255.255.255.128 (/25) splits the subnet incorrectly; hosts on the upper half (x.x.x.128–x.x.x.254) become unreachable from the lower half without a router.

5. Treating /31 and /32 as unusable. /31 is valid for point-to-point links (RFC 3021); /32 is the standard notation for host routes and loopback addresses — both are actively used in production networks.

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