1.7 Magic Number Subnetting

Question 1

What calculation helps determine the number of subnets and hosts per subnet?

Answer 1

Number of subnets = 2^subnet bits

Number of hosts per subnet = (2^host bits) - 2

Transcript: “2 to the second power… four networks… 2 to the sixth power minus 2… 62 hosts” (2:30–3:48)

Question 2

What are the four key addresses you need to calculate for a subnet?

Answer 2

The subnet ID, broadcast address, first host address, and last host address.

Network address/subnet ID
* The first address in the subnet

Broadcase address
* the last address in the subnet

First available host address
* one more than the network address

Last available host address
* one less than the broadcast address

Transcript: “network address… broadcast address… first available host address… last available host address” (4:16–4:39)

Question 3

What is the magic number in subnetting?

Answer 3

It’s calculated as 256 minus the interesting octet’s value in the subnet mask.

Transcript: “magic number, which is 256 minus the value of the interesting octet” (8:21–8:44)

Question 4

How is the subnet ID determined using the magic number method?

Answer 4

Find the range the IP falls into and take the first value of that range.

Transcript: “fit the 77 IP address in the range… first number in that range… 64” (11:01–11:50)

Question 5

How is the broadcast address calculated using the magic number?

Answer 5

Subnet ID + magic number - 1.

Transcript: “take the subnet ID… add the magic number and subtract 1” (13:20–13:49)

Question 6

How do you determine the first and last usable host addresses on a subnet?

Answer 6

First host = subnet ID + 1; Last host = broadcast address - 1.

Transcript: “calculate the first host… add 1… last usable… subtract 1” (14:07–14:23)

Question 7

What is an interesting octet in the magic number method?

Answer 7

The octet in the subnet mask that is neither 255 nor 0.

Transcript: “mask… not 0 and not 255… is our interesting octet” (10:28–10:39)

Question 8

How can a predefined chart help with subnetting using the magic number method?

Answer 8

It shows CIDR ranges, subnet masks, and magic numbers for faster calculation.

Transcript: “predefined chart… CIDR block notation… magic number… subnet mask” (18:15–19:00)

Question 9

Descibe the steps to use magic number subnetting

Answer 9

1. Convert the subnet mask to decimal (if necessary)
2. ID the interesting octet

neither 255 nor 0

3. Calculate the magic number

256 - the interesting octet

4. Calculate the host range
5. ID the network address
6. ID the broadcast address

Question 10

Outline the steps for identifying the subnet ID?

Answer 10

• If the mask is 255, copy the IP address from the mask down
• If the mask is 0, copy the zero
• ID the interesting octet
• ID the magic number
• ID where the host of the IP falls in the host ranges and put the first number

Question 11

Outline the steps to calculate the broadcast address

Answer 11

• If the mask is 255, copy the subnet ID
• If the mask is 0, write 255
• Anything not 255 or 0 is the interesting octet
• ID the magic number
• Calculate the subnet ID + magic number - 1