Subnet Mask IP & Notazione CIDR: /24, /16, /8 e oltre

Subnet masks and CIDR notation are fundamental to IP networking. Whether you are designing a cloud VPC, configuring a home router, or preparing for a networking exam, understanding how subnet masks divide IP addresses into network and host portions is essential. This guide covers everything from binary basics to a complete CIDR reference table.

1. What Is a Subnet Mask?

A subnet mask is a 32-bit number that separates an IP address into a network portion and a host portion. The network bits are set to 1, and the host bits are set to 0.

Binary breakdown of 255.255.255.0:

255  .  255  .  255  .  0
11111111.11111111.11111111.00000000
|-------- Network --------|- Host-|
        24 bits               8 bits

The subnet mask is applied via a bitwise AND operation with the IP address. The result is the network address. Bits set to 1 in the mask identify the network; bits set to 0 identify host addresses within that network.

Example: IP 192.168.1.100 with mask 255.255.255.0

IP Address:    192.168.1.100   = 11000000.10101000.00000001.01100100
Subnet Mask:   255.255.255.0   = 11111111.11111111.11111111.00000000
               ─────────────────────────────────────────────────────
Network (AND): 192.168.1.0     = 11000000.10101000.00000001.00000000
Broadcast:     192.168.1.255   = 11000000.10101000.00000001.11111111
Host Range:    192.168.1.1 — 192.168.1.254  (254 usable hosts)

2. CIDR Notation: /24, /16, /8

CIDR (Classless Inter-Domain Routing) notation appends a slash and the number of network bits to the IP address. It replaced the old classful addressing system (Class A, B, C) in 1993.

The /number indicates how many leading bits of the 32-bit address are the network prefix. The remaining bits are for host addresses.

192.168.1.0/24
│           │
│           └── 24 network bits (slash notation)
└── Network prefix

Binary view:
11000000.10101000.00000001 . 00000000
|---- 24 network bits ----| |8 host |

3. Complete CIDR Reference Table: /8 to /32

This table shows every CIDR prefix from /8 to /32 with the corresponding subnet mask, wildcard mask, total addresses, and usable host count.

* /31 uses both addresses per RFC 3021 (point-to-point). /32 is a single host route.

4. How Subnetting Works

An IPv4 address is 32 bits. The subnet mask determines which bits belong to the network and which to the host. Visualizing this in binary makes it clear:

Example: 172.16.10.0/20

IP:   10101100.00010000.00001010.00000000
Mask: 11111111.11111111.11110000.00000000
      |---- Network bits (1s in mask) ----|--- Host bits (0s in mask) ---|
      |<-------- 20 bits -------->|<- 12 bits ->|

Network:   172.16.0.0     (host bits all 0)
Broadcast: 172.16.15.255  (host bits all 1)
Range:     172.16.0.1 — 172.16.15.254
Hosts:     2^12 - 2 = 4,094

Key rules:

• The first address (all host bits = 0) is the network address.
• The last address (all host bits = 1) is the broadcast address.
• Usable hosts = 2^(host bits) - 2 (subtract network and broadcast).
• For /31 and /32, special rules apply (point-to-point links and single hosts).

5. Calculating Subnets: Step-by-Step

Given 10.0.0.0/22, find the network details:

10.0.0.0/22

Binary mask: 11111111.11111111.11111100.00000000 = 255.255.252.0
Wildcard:    00000000.00000000.00000011.11111111 = 0.0.3.255

Network:     10.0.0.0
First host:  10.0.0.1
Last host:   10.0.3.254
Broadcast:   10.0.3.255
Total:       1,024 addresses
Usable:      1,022 hosts

6. Private IP Ranges (RFC 1918)

These three IP ranges are reserved for private networks and are not routable on the public internet:

Additionally, 169.254.0.0/16 is used for link-local (APIPA) and 127.0.0.0/8 is the loopback range.

Private Ranges (RFC 1918):
┌─────────────────┬───────────────────────────┬──────────────┐
│  10.0.0.0/8     │  10.0.0.0 – 10.255.255.255│ 16,777,216   │
│ 172.16.0.0/12   │ 172.16.0.0 – 172.31.255.255│  1,048,576   │
│ 192.168.0.0/16  │ 192.168.0.0 – 192.168.255.255│     65,536 │
└─────────────────┴───────────────────────────┴──────────────┘

Other reserved:
127.0.0.0/8       Loopback
169.254.0.0/16    Link-local (APIPA)
100.64.0.0/10     Carrier-grade NAT (RFC 6598)

7. Common Subnets in Practice

Different environments use different subnet sizes:

# AWS VPC example
VPC:            10.0.0.0/16     (65,536 IPs)
├── Public-1a:  10.0.1.0/24     (256 IPs)
├── Public-1b:  10.0.2.0/24     (256 IPs)
├── Private-1a: 10.0.10.0/24    (256 IPs)
├── Private-1b: 10.0.11.0/24    (256 IPs)
├── DB-1a:      10.0.20.0/24    (256 IPs)
└── DB-1b:      10.0.21.0/24    (256 IPs)

# Kubernetes (default)
Pod CIDR:       10.244.0.0/16
Service CIDR:   10.96.0.0/12

8. IPv6 CIDR Basics

IPv6 addresses are 128 bits long, giving vastly larger address spaces. CIDR notation works the same way:

Unlike IPv4, IPv6 does not use broadcast addresses. The /64 boundary is significant because Stateless Address Autoconfiguration (SLAAC) requires it.

IPv6 Address: 2001:0db8:85a3:0000:0000:8a2e:0370:7334/64

|---- 64-bit Network Prefix ----|---- 64-bit Interface ID ----|
2001:0db8:85a3:0000              :0000:8a2e:0370:7334

Common allocations:
/32  →  ISP allocation       (65,536 × /48 sites)
/48  →  Site allocation      (65,536 × /64 subnets)
/64  →  Single subnet        (18,446,744,073,709,551,616 hosts)
/128 →  Single host          (loopback, host routes)

9. Supernetting (Route Aggregation)

Supernetting combines multiple smaller subnets into a single larger prefix to reduce routing table entries. This is also called route summarization or aggregation.

Example: Combining four /24 networks into one /22:

Before (4 routes):
192.168.0.0/24   →  192.168.0.0   – 192.168.0.255
192.168.1.0/24   →  192.168.1.0   – 192.168.1.255
192.168.2.0/24   →  192.168.2.0   – 192.168.2.255
192.168.3.0/24   →  192.168.3.0   – 192.168.3.255

After (1 route):
192.168.0.0/22   →  192.168.0.0   – 192.168.3.255

Binary proof:
192.168.0.0 = 11000000.10101000.000000|00.00000000
192.168.1.0 = 11000000.10101000.000000|01.00000000
192.168.2.0 = 11000000.10101000.000000|10.00000000
192.168.3.0 = 11000000.10101000.000000|11.00000000
                                 ^^^^^^
                            22 common bits → /22

Benefits: Smaller routing tables, faster lookups, reduced BGP announcements, and simpler ACLs.

Rule: Subnets must be contiguous and aligned to the supernet boundary. You cannot aggregate 192.168.1.0/24 and 192.168.5.0/24 into a single prefix.

10. Subnet Calculation Tools

These tools help you calculate subnets quickly:

# ipcalc example
$ ipcalc 192.168.1.0/24
Address:   192.168.1.0          11000000.10101000.00000001. 00000000
Netmask:   255.255.255.0 = 24   11111111.11111111.11111111. 00000000
Wildcard:  0.0.0.255            00000000.00000000.00000000. 11111111
Network:   192.168.1.0/24
HostMin:   192.168.1.1
HostMax:   192.168.1.254
Broadcast: 192.168.1.255
Hosts/Net: 254

# Python ipaddress
import ipaddress
net = ipaddress.ip_network('10.0.0.0/22')
print(net.network_address)   # 10.0.0.0
print(net.broadcast_address) # 10.0.3.255
print(net.num_addresses)     # 1024
print(list(net.subnets()))   # [10.0.0.0/23, 10.0.2.0/23]

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