Overview
This lecture explains the 7-second (7c) subnetting shortcut, a method to quickly determine network, broadcast, and usable IP addresses using a pre-made chart, avoiding manual binary or complex math.
Introduction to 7c Subnetting
- 7c subnetting is a shortcut method for subnet calculations that doesn't require manual math or binary conversion.
- The process relies on building and referencing a prepared chart covering subnet masks, address ranges, and network boundaries.
- The only math required is simple addition and subtraction for first and last usable IP addresses.
Building the Subnetting Chart
- Chart includes columns for subnet mask (CIDR and decimal), number of networks, and addresses per subnet.
- Networks column starts at 2 and doubles until 256; addresses start at 128 and halve down to 1.
- A boundaries chart lists subnet start points (e.g., 0, 64, 128 for 64-host subnets).
- The chart enables instant conversion between CIDR and decimal mask.
Four-Step 7c Subnetting Process
- Step 1: Convert CIDR notation to decimal subnet mask using the chart.
- Step 2: Determine subnet (network) address by applying mask logic (255 = bring address, 0 = 0).
- Step 3: Find broadcast address (255 = bring address, 0 = 255; use chart for custom ranges).
- Step 4: First usable IP = network address + 1; last usable IP = broadcast address - 1.
Subnetting Examples
- Examples show converting /24, /26, /20, /11, and /17 subnet masks to decimal using the chart.
- Determining which subnet block an address belongs to (using ranges like 0–63, 64–127, etc.).
- Applying steps to extract network address, broadcast address, and usable IP range for each scenario.
Tips and Recommendations
- Write or type out the chart before starting an exam for quick reference.
- Consider printing subnet boundaries for quick lookup during calculations.
- Ensure proper supplies (e.g., fine-tip marker) for creating legible charts in a physical testing environment.
- Choose the subnetting shortcut method that works best for you.
Key Terms & Definitions
- CIDR (Classless Inter-Domain Routing) — Notation for subnetting (e.g., /24) indicating bits in the subnet mask.
- Subnet Mask — A value identifying the network portion of an IP address.
- Network Address — The first address in a subnet, used to identify the network.
- Broadcast Address — The last address in a subnet, used for broadcasting to all hosts.
- Usable IP Range — Addresses between the network and broadcast addresses, assignable to devices.
Action Items / Next Steps
- Practice building the subnetting chart from memory before exams.
- Try using the chart to solve sample subnetting problems.
- Prepare appropriate writing tools for test day as permitted.