1.7 Seven Second Subnetting

Question 1

What is the seven second subnetting shortcut?

Answer 1

A method that allows subnet calculations without math, using a predefined chart to quickly determine subnet, broadcast, and host addresses.

Transcript: “This shortcut is the seven second subnetting shortcut… there is no math involved… everything you need will be predefined in the chart…” (0:23–1:02)

Question 2

What are the four main steps of the seven second subnetting process?

Answer 2

1. Convert the subnet mask to decimal
2. Determine the subnet address
3. Identify the broadcast address
4. Calculate first and last usable IPs.

Transcript: “The seven second subnetting process is a four-step process…” (3:40–4:27)

Question 3

How do you calculate the network address?

Answer 3

If mask is 255, bring down the address; if mask is 0, use the 0

Transcript: “If the mask is 255, we bring down the address… if the mask is 0, we use 255…” (5:09–6:10)

Question 4

How is the first usable IP address determined in this shortcut method?

Answer 4

By adding 1 to the network (subnet) address.

Transcript: “To calculate the first IP, we take the network address and we add 1.” (6:19–6:23)

Question 5

How do you calculate the broadcast address?

Answer 5

If mask if 255, bring down the address; if mask is 0, use 255

Question 6

How do you calculate the network ID and broadcast address when subnetting doesn’t fall on simple 8-bit boundaries?

Answer 6

For network ID, use the chart to find which subrange the IP address falls into and use the number that is the beginning of that range.

For broadcast ID, use the chart to find the next block start, subtract 1 to find the broadcast address.

Transcript: “We know the dot 88 fits into that second block… the next block starts with 128, so the broadcast address is 127.” (8:48–9:07)