Subnetting Cheatsheet: Sizes, Masks, and Conversions

You’re designing a network. You need a /22 subnet but you don’t remember offhand how many hosts that is. Or you’re given a netmask 255.255.252.0 and need to translate to CIDR. Or you’re sizing a cloud VPC and need to plan subnets.

This post is the cheatsheet. Prefix length → mask → host count, all the common sizes, in one place, with the conversions you’ll actually need.

The Prefix Length Table

IPv4 prefix lengths, in order:

CIDR	Netmask	Wildcard	Total addresses	Usable hosts	Common use
/32	255.255.255.255	0.0.0.0	1	1	Single host
/31	255.255.255.254	0.0.0.1	2	2*	Point-to-point links
/30	255.255.255.252	0.0.0.3	4	2	Point-to-point
/29	255.255.255.248	0.0.0.7	8	6	Small VLAN
/28	255.255.255.240	0.0.0.15	16	14	Small DMZ
/27	255.255.255.224	0.0.0.31	32	30	Small office subnet
/26	255.255.255.192	0.0.0.63	64	62	Medium subnet
/25	255.255.255.128	0.0.0.127	128	126	Half a /24
/24	255.255.255.0	0.0.0.255	256	254	Standard LAN
/23	255.255.254.0	0.0.1.255	512	510	Larger LAN
/22	255.255.252.0	0.0.3.255	1,024	1,022	Building network
/21	255.255.248.0	0.0.7.255	2,048	2,046	Campus subnet
/20	255.255.240.0	0.0.15.255	4,096	4,094	Medium VPC
/19	255.255.224.0	0.0.31.255	8,192	8,190	Large VPC
/18	255.255.192.0	0.0.63.255	16,384	16,382	Very large LAN
/17	255.255.128.0	0.0.127.255	32,768	32,766	Massive subnet
/16	255.255.0.0	0.0.255.255	65,536	65,534	Class B equivalent
/15	255.254.0.0	0.1.255.255	131,072	131,070	Multiple /16s
/14	255.252.0.0	0.3.255.255	262,144	262,142	Large org
/13	255.248.0.0	0.7.255.255	524,288	524,286	Huge org
/12	255.240.0.0	0.15.255.255	1,048,576	1,048,574	RIR allocation
/8	255.0.0.0	0.255.255.255	16,777,216	16,777,214	Class A equivalent

* /31 in RFC 3021 mode allows 2 usable hosts on point-to-point links (no network/broadcast).

For the underlying concepts of CIDR notation and netmasks, see subnet mask CIDR explained.

Why “Usable Hosts” Is Less Than “Total Addresses”

In standard IPv4, each subnet reserves:

• Network address (first address) — Identifies the subnet itself.
• Broadcast address (last address) — Sends to all hosts on the subnet.

So a /24 with 256 addresses has 254 usable hosts: 192.168.1.0 (network) and 192.168.1.255 (broadcast) are reserved; 192.168.1.1-254 are usable.

Exceptions:

• /31 (RFC 3021): for point-to-point links. No broadcast needed; both addresses usable.
• /32: single host; concept of network/broadcast doesn’t apply.

Common Cloud Subnet Choices

AWS VPC sizing

• VPC: /16 to /28. Default new VPCs are /16.
• Subnets within VPC: /16 to /28. Each subnet must be in one Availability Zone.
• Common subnet size: /24 (256 addresses) or /20 (4096 addresses).
• AWS reserves 5 addresses per subnet for AWS internal use, in addition to the network/broadcast.

So a /24 in AWS gives you 251 usable host IPs.

GCP and Azure

Similar conventions. GCP subnets are typically /24 to /20. Azure subnets reserve 5 addresses too.

Kubernetes pod CIDRs

• /14 for the whole cluster (4096 pods × 4096 hosts).
• /22 to /24 per node (pods on that node).

Specific numbers vary by CNI plugin; the orders of magnitude are similar.

Converting Between Forms

CIDR to host count

hosts = 2^(32 - cidr) - 2 (for cidr ≤ 30).

/22 → 2^10 - 2 = 1022 usable hosts.

Host count to CIDR

Round up to next power of 2, then cidr = 32 - log2(hosts + 2).

Need 100 hosts → 128 addresses → /25 (with 126 usable hosts).

Netmask to CIDR

Count the 1-bits in the binary netmask.

255.255.252.0 = 11111111.11111111.11111100.00000000 = 22 ones → /22.

CIDR to netmask

Number of 1-bits then 0-bits, packed into octets.

/22 = 22 ones then 10 zeros → 11111111.11111111.11111100.00000000 → 255.255.252.0.

Calculating Network Address from IP + CIDR

To find which subnet an IP belongs to:

IP:     203.0.113.42
CIDR:   /27 (224 in last octet)

42 / 32 = 1 remainder 10
Network: 203.0.113.32 (first multiple of 32 ≤ 42)
Range: 203.0.113.32 - 203.0.113.63

Or programmatically with ipaddress in Python:

import ipaddress
net = ipaddress.ip_network('203.0.113.42/27', strict=False)
print(net.network_address)  # 203.0.113.32
print(net.broadcast_address) # 203.0.113.63

Subnet Adjacency

For multi-region networks where you need non-overlapping CIDRs:

A /16 covers 65536 addresses. Splitting into /24s gives you 256 /24 subnets. Splitting into /20s gives 16 /20 subnets.

Example allocation plan for a multi-region deployment:

us-east:   10.0.0.0/16   (10.0.0.0 - 10.0.255.255)
us-west:   10.1.0.0/16   (10.1.0.0 - 10.1.255.255)
eu-west:   10.2.0.0/16   (10.2.0.0 - 10.2.255.255)
ap-south:  10.3.0.0/16   (10.3.0.0 - 10.3.255.255)

Each /16 can be subdivided into /20s, /24s, etc., for AZ subnets within the region.

Variable Length Subnet Masking (VLSM)

You don’t have to give every subnet the same size. VLSM lets you allocate efficiently:

Building network: 192.168.0.0/22  (1024 addresses)
  HR (50 users):  192.168.0.0/26  (64 addresses)
  Engineering (200): 192.168.0.64/25  (128 addresses)
  Wait — overlaps. Better:
  Engineering: 192.168.1.0/24 (256 addresses)
  Sales (100): 192.168.2.0/25 (128 addresses)
  Servers: 192.168.3.0/24 (256 addresses)

VLSM is standard practice in 2026. Modern routers handle arbitrary prefix lengths.

IPv6 Sizing

IPv6 uses different conventions:

• /128 — single host.
• /64 — standard subnet (auto-config requires this).
• /56 — typical home assignment (256 /64 subnets).
• /48 — typical enterprise allocation (65,536 /64 subnets).
• /32 — typical LIR allocation (4 billion /64 subnets).

The IPv6 world doesn’t conserve addresses the way IPv4 does. You always use /64 for end subnets; you delegate /48 or /56 to sites. See IPv6 deployment guide.

Practical Sizing Guide

When designing a new network, ask:

1. How many hosts in the largest subnet? Pick the smallest CIDR that fits.
2. How many subnets total? Multiply.
3. Growth? Add 2-4x headroom.
4. Multi-region? Make sure regions don’t overlap.
5. Future VPN peering? Avoid common ranges (10.0.0.0/24, 192.168.1.0/24).

A typical “design a corporate network for 500 employees in 3 offices” looks like:

Corp HQ:       10.10.0.0/16 (potential 65k hosts; we'll use a fraction)
  Engineering: 10.10.10.0/23 (510 hosts)
  Sales:       10.10.20.0/24 (254 hosts)
  Servers:     10.10.30.0/24 (254 hosts)
  Wi-Fi:       10.10.40.0/22 (1022 hosts)

Branch 1:      10.20.0.0/16
Branch 2:      10.30.0.0/16

Each office has plenty of room within its /16. Inter-office VPN connects them without overlap.

Tools

A few practical tools for working with subnets:

• ipcalc — CLI tool. ipcalc 192.168.0.0/22 gives all the details.
• sipcalc — More features than ipcalc.
• Python ipaddress — Programmatic subnet math.
• Online calculators — subnet-calculator.com, ipcalc.co.uk.

TL;DR

• Reference the table for prefix length → mask → host count.
• Each subnet reserves network + broadcast; usable hosts = total − 2.
• AWS reserves 5 additional addresses per subnet.
• /31 for point-to-point (RFC 3021); both addresses usable.
• VLSM lets you mix subnet sizes for efficient allocation.
• For multi-region, give each region a non-overlapping /16 or /20.
• For future VPN peering, avoid common ranges that everyone else picks.
• IPv6 uses /64 standard subnets; allocate /48 or /56 to sites.

Subnetting is one of those skills that gets fluent with practice. The table above covers 95% of what comes up in modern network design. For the underlying concepts, see subnet mask CIDR explained; for the private ranges most of your networks will live in, private IP ranges RFC 1918.