Subnetting Cheat Sheet

For CCNA 200-301 candidates. One page. Print and keep on your desk while you study.

1. Powers of Two (memorize columns 0-8)

2. Subnet Mask Table (the ten you'll see most)

3. Core Formulas

• Hosts per subnet = 2^h − 2 (where h = host bits = 32 − prefix)
• Subnets per network = 2^s (where s = subnet bits borrowed from the parent network)
• Block size = 256 − interesting octet of mask
• Wildcard mask = 255.255.255.255 − subnet mask (per octet)

4. Fast Method (do this in your head)

Given an IP + mask:

1. Find the interesting octet, the rightmost octet of the mask that is not 0 or 255 (e.g., /27 → 4th octet = 224). 2. Block size = 256 − interesting octet. So /27 → 256 − 224 = 32. 3. Subnet boundaries = multiples of the block size in the interesting octet (0, 32, 64, 96, 128, 160, 192, 224). 4. Find the subnet the host IP lives in: the largest multiple of the block size ≤ that host's interesting-octet value. 5. Network address = subnet boundary. 6. Broadcast = next subnet boundary − 1. 7. Usable range = network + 1 → broadcast − 1.

5. Worked Examples

Example A: 172.16.45.50 /26

• Interesting octet (4th) of mask: 192. Block size = 256 − 192 = 64.
• Boundaries: 0, 64, 128, 192.
• 50 falls into the 0 subnet.
• Network: 172.16.45.0 · Broadcast: 172.16.45.63 · Hosts: .1-.62.

Example B: 10.1.20.130 /27

• Interesting octet: 224. Block size = 32.
• Boundaries: 0, 32, 64, 96, 128, 160, 192, 224.
• 130 falls into the 128 subnet.
• Network: 10.1.20.128 · Broadcast: 10.1.20.159 · Hosts: .129-.158.

Example C: 192.168.5.200 /29

• Interesting octet: 248. Block size = 8.
• Boundaries: ..., 192, 200, 208, ...
• 200 falls into the 200 subnet (exactly on the boundary).
• Network: 192.168.5.200 · Broadcast: 192.168.5.207 · Hosts: .201-.206.

Example D: Point-to-point WAN: 10.10.10.0 /30

• Block size = 4. Subnets: .0, .4, .8, .12 ...
• For the .0 subnet: Network = .0 · Broadcast = .3 · Hosts = .1 and .2.

Example E: /25 split of 192.168.1.0/24

• Block size 128.
• 192.168.1.0/25 (hosts .1-.126, broadcast .127)
• 192.168.1.128/25 (hosts .129-.254, broadcast .255)

6. VLSM Design (allocate largest blocks first)

For a 200-host LAN + a 60-host LAN + a 28-host LAN + a 2-host WAN out of 10.0.0.0/24:

Realistic VLSM out of 10.0.0.0/23 (510 hosts available):

Rule of thumb: sort networks by size, biggest first, then walk through the address space.

7. Wildcard Masks (for ACLs)

Wildcard = inverse of subnet mask. For /27 (mask 255.255.255.224):

Wildcard = 0.0.0.31

Used in Cisco ACLs and OSPF network statements:

access-list 10 permit 192.168.5.32 0.0.0.31, matches 192.168.5.32-.63.

8. IPv4 Private Ranges (RFC 1918)

9. Common Mistakes to Avoid

• Confusing block size with number of hosts, block size includes network + broadcast.
• Forgetting that /31 has 0 usable hosts under RFC 3021 (still used for point-to-point WAN links).
• Mixing up subnet mask and wildcard mask in ACLs, they are inverses.
• Counting the broadcast address as a usable host.

_Authored by CISNET instructors. Print this page (Cmd/Ctrl + P → Save as PDF) for offline reference._