🎯 New! Master certifications with Performance-Based Questions (PBQ) — realistic hands-on practice for CompTIA & Cisco exams!

AiCybr
🧮

Subnetting Cheat Sheet

IPv4 subnet masks, CIDR notation, and host calculations

🔌 Network Ports📚 OSI Model🧮 Subnetting🔢 IPv4 vs IPv6🛜 Routing Protocols

📐 Hosts Formula

2n - 2

n = host bits (32 - prefix)

🔢 Subnets Formula

2n

n = borrowed bits

🎭 Wildcard Formula

255 - subnet octet

Inverse of subnet mask

25 subnet entries • Class A: /8 • Class B: /16 • Class C: /24
CIDR Subnet Mask Usable Hosts Wildcard Class/Size Binary Mask
/32255.255.255.25510.0.0.0Host11111111.11111111.11111111.11111111
/31255.255.255.25420.0.0.1P2P Link11111111.11111111.11111111.11111110
/30255.255.255.25220.0.0.3P2P Link11111111.11111111.11111111.11111100
/29255.255.255.24860.0.0.7Small11111111.11111111.11111111.11111000
/28255.255.255.240140.0.0.15Small11111111.11111111.11111111.11110000
/27255.255.255.224300.0.0.31Small11111111.11111111.11111111.11100000
/26255.255.255.192620.0.0.63Small11111111.11111111.11111111.11000000
/25255.255.255.1281260.0.0.127Medium11111111.11111111.11111111.10000000
/24255.255.255.02540.0.0.255C11111111.11111111.11111111.00000000
/23255.255.254.05100.0.1.255Medium11111111.11111111.11111110.00000000
/22255.255.252.010220.0.3.255Medium11111111.11111111.11111100.00000000
/21255.255.248.020460.0.7.255Medium11111111.11111111.11111000.00000000
/20255.255.240.040940.0.15.255Large11111111.11111111.11110000.00000000
/19255.255.224.081900.0.31.255Large11111111.11111111.11100000.00000000
/18255.255.192.0163820.0.63.255Large11111111.11111111.11000000.00000000
/17255.255.128.0327660.0.127.255Large11111111.11111111.10000000.00000000
/16255.255.0.0655340.0.255.255B11111111.11111111.00000000.00000000
/15255.254.0.01310700.1.255.255Large11111111.11111110.00000000.00000000
/14255.252.0.02621420.3.255.255Large11111111.11111100.00000000.00000000
/13255.248.0.05242860.7.255.255Large11111111.11111000.00000000.00000000
/12255.240.0.010485740.15.255.255Large11111111.11110000.00000000.00000000
/11255.224.0.020971500.31.255.255Large11111111.11100000.00000000.00000000
/10255.192.0.041943020.63.255.255Large11111111.11000000.00000000.00000000
/9255.128.0.083886060.127.255.255Large11111111.10000000.00000000.00000000
/8255.0.0.0167772140.255.255.255A11111111.00000000.00000000.00000000

⭐ Most Common Subnets

/30 255.255.255.252 2 hosts
/29 255.255.255.248 6 hosts
/28 255.255.255.240 14 hosts
/27 255.255.255.224 30 hosts
/26 255.255.255.192 62 hosts
/25 255.255.255.128 126 hosts
/24 255.255.255.0 254 hosts
/16 255.255.0.0 65534 hosts
/8 255.0.0.0 16777214 hosts

🎯 Exam Tips

  • • Memorize /24, /25, /26, /27, /28, /29, /30
  • • /30 = 2 usable hosts (point-to-point links)
  • • Subtract 2 from total for usable hosts (network + broadcast)
  • • Binary powers: 128, 64, 32, 16, 8, 4, 2, 1

🧠 Quick Memory Trick

/24 = 256 total, 254 usable (most common)

/25 = half of /24 = 128 total, 126 usable

/26 = half of /25 = 64 total, 62 usable

Pattern: Each increment halves the addresses