Why This Cheat Sheet Matters for CCNA
This cheat sheet covers the most important IPv4 Subnetting concepts tested on the CCNA (CCNA) certification exam. It contains 3 sections with 17 key points that you should memorize before exam day. Use this as a quick-reference guide during your final review sessions.
3Sections
17Key Points
Subnet Mask Basics
- /24 = 255.255.255.0 → 256 addresses, 254 usable hosts.
- /25 = 255.255.255.128 → 128 addresses, 126 usable hosts.
- /26 = 255.255.255.192 → 64 addresses, 62 usable hosts.
- /27 = 255.255.255.224 → 32 addresses, 30 usable hosts.
- /28 = 255.255.255.240 → 16 addresses, 14 usable hosts.
- /30 = 255.255.255.252 → 4 addresses, 2 usable hosts (point-to-point links).
Subnetting Steps
- 1. Determine how many subnets or hosts are needed.
- 2. Calculate the number of bits to borrow (2^n ≥ required subnets).
- 3. New subnet mask = original mask + borrowed bits.
- 4. Block size = 256 − last non-zero octet of the subnet mask.
- 5. First address = network address. Last address = broadcast. Usable = everything in between.
- Usable hosts per subnet = 2^(host bits) − 2.
Private IP Ranges
- Class A: 10.0.0.0 – 10.255.255.255 (/8)
- Class B: 172.16.0.0 – 172.31.255.255 (/12)
- Class C: 192.168.0.0 – 192.168.255.255 (/16)
- Loopback: 127.0.0.0/8 (127.0.0.1 is localhost).
- APIPA: 169.254.0.0/16 (auto-assigned when DHCP fails).
Practice IPv4 Subnetting Questions
Put your knowledge to the test with practice questions.
Cisco Command Quick Reference
Cisco IOS commands follow a hierarchical structure: User EXEC mode (>), Privileged EXEC mode (#), Global Configuration mode (config)#, and Interface Configuration mode (config-if)#. Master the transitions between these modes and the key "show" commands for each technology area — they are heavily tested on every Cisco exam.