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The Enhanced Bob Maneuver for Subnetting (Or How to Subnet Anything in Under a Minute) Legend has it that once upon a time a networking instructor named Bob taught a class of students a method of subnetting any address using a special chart. This was known as the Bob Maneuver. These students, being the smart type that networking students usually are, added a row to the top of the chart, and the Enhanced Bob Maneuver was born. The chart and instructions on how to use it follow. With practice, you should be able to subnet any address and come up with an IP plan in under a minute. After all, it’s just math!

The Enhanced Bob Maneuver 128 8

192 64 7

224 32 6

240 16 5

248 8 4

252 4 3

254 2 2

255 1 1

126 62 30 14 6 4 N/A Suppose that you have a Class C network and you need nine subnets. 1. 2. 3. 4.

5.

6.

Subnet Mask Target Number Bit Place Number of Valid Subnets

On the bottom line (Number of Valid Subnets), move from right to left and find the closest number that is bigger than or equal to what you need: Nine subnets—move to 14. From that number (14), move up to the line called Bit Place. Above 14 is bit place 4. The dark line is called the high-order line. If you cross the line, you have to reverse direction. You were moving from right to left; now you have to move from left to right. Go to the line called Target Number. Counting from the left, move over the number of spaces that the bit place number tells you. Starting on 128, moving 4 places takes you to 16. This target number is what you need to count by, starting at 0, and going until you hit 255 or greater. Stop before you get to 256: Network Network Network Network 0—Network 16—Network 32—Network 48—Network 64—Network 80—Network 96—Network 112—Network 128—Network 144—Network 160—Network 176—Network 192—Network 208—Network 224—Network 240—Network 256 Stop—too far! These numbers are your network numbers. Expand to finish your plan. Network # Range of Valid Hosts Broadcast Numbers 0 1-14 15 16 17-30 31 32 33-46 47 48 49-62 63 64 65-78 79 80 81-94 95 96 97-110 111 112 113-126 127

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128 129-142 143 144 145-158 159 160 161-174 175 176 177-190 191 192 193-206 207 208 209-222 223 224 225-238 239 240 241-254 255 Notice that there are 14 subnets created from .16 to .224. Go back to the Enhanced Bob Maneuver chart and look above your target number to the top line. The number above your target number is your subnet mask. Above 16 is 240. Because you started with a Class C network, the new subnet mask is 255.255.255.240.

Now, let’s practice sub-netting with Enhanced Bob Maneuver. Let’s we want to have 4 subnets, how do we do it? a. On the bottom line (Number of Valid Subnets), move from right to left and find the closest number that is bigger than or equal to what you need: Four subnets—move to 6. b. From that number (6), move up to the line called Bit Place. Above 6 is bit place 3. c. The dark line is called the high-order line. If you cross the line, you have to reverse direction. You were moving from right to left; now you have to move from left to right. d. Go to the line called Target Number. Counting from the left, move over the number of spaces that the bit place number tells you. Starting on 128, moving 4 places takes you to 16. e. This target number is what you need to count by, starting at 0, and going until you hit 255 or greater. Stop before you get to 256: Network Network Network Network Network 0 8 --------248 252 f. These numbers are your network numbers. Expand to finish your plan. Network # Range of Valid Hosts Broadcast Numbers 0 1—7 8 ------------------------248 249—254 255 Notice that there are 6 subnets created from .8 to .248. g. Go back to the Enhanced Bob Maneuver chart and look above your target number to the top line. The number above your target number is your subnet mask. Above 8 is 248. Because you started with a Class C network, the new subnet mask is 255.255.255.248. Instructions: (a) Right, UP, Left, UP, and UP (EBM) (b) Right, UP, UP-Right, UP, UP, and UP (SEBM) 126 128

62 192 64

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30 224 32

14 240 16

6 248 8

2 252 4

0 254 2

N/A 255 1

No. of Valid Subnets Subnet Mask Target Number 2

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7

6

5

4

3

2

1

Bit Value Bit Place

126

62

30

14

6

4

N/A

No. of Valid Subnets

27 = 128

8

26 = 64

25 = 32

24 = 16

23 = 8

22 = 4

21 = 2

20 = 1

A Different Explanation You can call it a Super Enhanced Bob Maneuver. Do not get confused with the chart. Start from the bottom most line from right to left “No. of Valid Subnets”, move up to the “Bit Place” exactly the one above the subnets you needed, then look for the bit in bit value row. The bit value always come to the right of the bit place. Now, move up, up, and up, and there you did the subnetting. Remember the directions: 1. 2. 3. 4.

From the row “Number of Valid Subnets” (bottom line) move: ← (Right) From the bottom row move: ↑ (UP) to the cell right about the bottom line marked cell. From the “Bit Place” row move: ↑ (UP) and then move ← (Right) but just one cell. From the “Bit Value” row move: ↑ (UP), ↑ (UP), and ↑ (UP).

CIDR (/) C B A 1 2 128 126 .128 /25 /17 /9 2 4 64 62 .192 /26 /18 /10 3 8 32 30 .224 /27 /19 /11 4 16 16 14 .240 /28 /20 /12 5 32 8 6 .248 /29 /21 /13 6 64 4 2 .252 /30 /22 /14 7 128 2 0 .254 /31 /23 /15 8 255 1 N/A .255 /32 /24 /16 NOTE: Left side of the chart only shows class C’s details. Red columns in the chart above means limit is exceeded. Bit

Subnets

Total Hosts

Valid Hosts

Subnet-Mask

Class A’s default CIDR is /8, but when we borrow 8-bit for subnetting the CIDR changes to /16, and when borrow another 8-bit for subnetting the CIDR changes to /24, and after that we can only borrow 6 more bits, and the CIDR becomes /30. We cannot exceed this limit, because we at least have to have 2-host for each for hosts, broadcast, and network. What would we do if we’ve a class B or A? So, don’t you worry about that, because Cisco is never going to ask you for a thousand of hosts! But, actually Enhanced Bob Maneuver helps you with you 438-host. Let’s see how:

Given: 255.255.254.0 or /23 I hope you know that to which class it belongs, right? Yeah! It belongs to class B. let’s have a look at the Squeezed Bob Maneuver below: 128 8

192 7

224 6

240 5

248 4

252 3

254 2

255 1

Subnet Mask Bit Place

Red bits are borrowed, which lefts 9-bit in a class B for subnetting. Now, you also know that the 29 = 512. It’s easy, isn’t it? Let’s take another example of 1000 hosts.

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Given: 255.255.252.0 or /22 128 8

192 7

224 6

240 5

248 4

252 3

254 2

255 1

Subnet Mask Bit Place

Again 210 = 1024. It’s all about math, if you know how to work with binary I hope you can then work with subnetting.

Always Remember: 1. 2. 3. 4.

The number of hosts required must be equal (+) or less than (≤) to the next network. Remember the power of 2 (binary). Start with zero Add the increment (next network number) to the preceding network. For example: 0 as the first network (in example above) and the next network would be 1024. 0 + 1024 = 1024 1024 + 1024 = 2048 and so on. 5. Subnet mask would remain same for all the networks with /23. For example: a. Network: 172.16.0.0/23 b. Network: 172.16.4.0/23 Net-ID: 172.16.0.0 Net-ID: 172.16.4.0 First IP: 172.16.0.1 First IP: 172.16.4.1 Last IP: 172.16.3.254 Last IP: 172.16.7.254 B.C-ID: 172.16.7.255 B.C-ID: 172.16.3.255 And so on.

Easy Remember Super Enhanced Bob Maneuver

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This is how SEBM (Super Enhanced Bob Maneuver) is explained. I hope it’s been informative for you. For more information keep visiting: http://vx-ccna-fm.blogspot.com.

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