IP Addressing &
Subnetting Cheatsheet

🌐 1. IPv4 Basics

An IPv4 address is a 32-bit number written as four decimal octets separated by dots, for example 192.168.1.1. Each octet is 8 bits, giving values from 0 to 255.

Every IP address has two logical parts — the network portion (identifies the network) and the host portion (identifies the device on that network). The subnet mask determines where the boundary falls.

# Example: 192.168.1.100 / 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 bits (24)    Host bits (8)

Network    : 192.168.1.0    # all host bits → 0
Broadcast  : 192.168.1.255  # all host bits → 1
Usable IPs : 192.168.1.1 → 192.168.1.254  # 254 hosts

📋 2. IP Classes (A – E)

Classful addressing divides IPv4 space into five classes based on the leading bits of the first octet.

⚠️ 127.x.x.x is reserved for loopback (127.0.0.1). Classful addressing is largely superseded by CIDR but remains essential knowledge.

🔒 3. Public vs Private IP Addresses

Private IP ranges are defined in RFC 1918 and are not routable on the public internet. They allow organisations to reuse address space internally, with NAT (Network Address Translation) mapping them to a public IP at the gateway.

# RFC 1918 — Private address ranges

Class A  10.0.0.0    – 10.255.255.255   # /8  — 16,777,216 addresses
Class B  172.16.0.0  – 172.31.255.255   # /12 —  1,048,576 addresses
Class C  192.168.0.0 – 192.168.255.255  # /16 —     65,536 addresses

# Other special ranges
127.0.0.0/8    # Loopback (localhost)
169.254.0.0/16 # APIPA / link-local (no DHCP available)
0.0.0.0/8      # "This" network (source only)

NAT in brief: a router replaces the private source IP with its own public IP before forwarding packets to the internet, then reverses the translation for return traffic. This conserves public address space and adds a layer of obscurity.

✂️ 4. Subnetting Basics

Subnetting divides a larger network into smaller, logically separated segments. Two equivalent notations exist:

IP   : 192.168.1.0
Mask : 255.255.255.0

# 255 = all 8 bits on
# 0   = all 8 bits off

IP/prefix: 192.168.1.0/24

# /24 = 24 network bits
# 32 - 24 = 8 host bits
# 2^8 - 2 = 254 usable hosts

The /prefix (CIDR prefix length) tells you how many of the 32 bits are used for the network portion. Subtracting from 32 gives the number of host bits. Usable hosts = 2^{host bits} − 2 (subtract 1 for the network address and 1 for broadcast).

📊 5. Subnet Quick-Reference Table

Common CIDR prefixes from /8 to /30 with their subnet masks and usable host counts.

🧮 6. Subnetting — Step-by-Step Example

Given: 192.168.10.55 /26 — find the network, broadcast, and usable host range.

# Summary for 192.168.10.55 /26
Network Address : 192.168.10.0
Subnet Mask     : 255.255.255.192  (/26)
Broadcast       : 192.168.10.63
First Usable IP : 192.168.10.1
Last Usable IP  : 192.168.10.62
Usable Hosts    : 62

⚡ Interactive Subnet Calculator

Enter an IP address and a subnet mask (dotted-decimal or CIDR prefix) to instantly calculate all network values.