Discovering All Subnets Back Application SLSM: 13 Questions
This area covers the added chic of IP acclamation problems mentioned in the accession to
this appendix. The catechism is as follows:
Assuming SLSM, what are the subnets of this network?
For practice, acknowledgment that catechism for the afterward networks and masks:
1. 10.0.0.0, affectation 255.192.0.0
2. 10.0.0.0, affectation 255.224.0.0
3. 10.0.0.0, affectation 255.248.0.0
4. 10.0.0.0, affectation 255.252.0.0
5. 10.0.0.0, affectation 255.255.128.0
6. 10.0.0.0, affectation 255.255.192.0
7. 172.31.0.0, affectation 255.255.224.0
8. 172.31.0.0, affectation 255.255.240.0
9. 172.31.0.0, affectation 255.255.252.0
10. 172.31.0.0, affectation 255.255.255.224
11. 192.168.15.0, affectation 255.255.255.192
Table D-76 Catechism 25: Subnet, Broadcast, Aboriginal and Aftermost Addresses Affected Application Subnet Chart
Octet 1 Octet 2 Octet 3 Octet 4
Address 10 1 1 1
Mask 255 0 0 0
Network cardinal 10 0 0 0
First accurate abode 10 0 0 1
Broadcast 10 255 255 255
Last accurate abode 10 255 255 254
IP Acclamation Convenance 47
12. 192.168.15.0, affectation 255.255.255.224
13. 192.168.15.0, affectation 255.255.255.240
These questions are mostly a subset of the aforementioned 25 subnetting questions covered in the aboriginal section
of this appendix. The explanations of the answers will be based on the seven-step algorithm from
Chapter 4, afresh actuality for convenience. Also, accumulate in apperception that this academic algorithm assumes
that the subnet acreage is 8 $.25 in breadth or less. However, some problems in this addendum accept a
longer subnet field. For those problems, the acknowledgment explains how to aggrandize the argumentation in this
baseline algorithm.
Step 1 Abode bottomward the classful arrangement number.
Step 2 For the aboriginal (lowest numeric) subnet number, archetype the absolute network
number. That is the aboriginal subnet number, and is additionally the aught subnet.
Step 3 Decide which octet contains the absolute subnet field; alarm this octet the
interesting octet. (Remember, this algorithm assumes 8 subnet $.25 or less.)
Step 4 Account the abracadabra cardinal by adding the mask’s absorbing octet
value from 256.
Step 5 Archetype bottomward the antecedent subnet number’s noninteresting octets assimilate the
next band as the abutting subnet number; alone one octet is missing at this point.
Step 6 Add the abracadabra cardinal to the antecedent subnet’s absorbing octet, and write
that bottomward as the abutting subnet number’s absorbing octet, commutual the next
subnet number.
Step 7 Echo Accomplish 5 and 6 until the new absorbing octet is 256. That subnet is
not valid. The ahead affected subnet is the aftermost accurate subnet, and also
the advertisement subnet.
Question 1: Answer
This catechism begins with the afterward basal facts:
Network 10.0.0.0
Mask 255.192.0.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 2nd
Magic number: 256 – 192 = 64
From there, Table D-77 shows the blow of the accomplish for the process.
48 CCIE Routing and Switching Exam Certification Guide
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Note that the advertisement subnet cardinal may not accept been accessible until attempting to abode the
final (invalid) abutting subnet number, as apparent in the aftermost row of the table. You can chase the steps
shown in the table, alive that back the absorbing octet’s amount is 256, you accept gone too far.
The advertisement subnet is the subnet that was begin one footfall prior.
Alternately, you can acquisition the advertisement subnet based on the afterward fact: the advertisement subnet’s
interesting octet is according to the subnet affectation amount in that aforementioned octet.
Question 2: Answer
This catechism begins with the afterward basal facts:
Network 10.0.0.0
Mask 255.224.0.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 2nd
Magic number: 256 – 224 = 32
From there, Table D-78 shows the blow of the accomplish for the process.
Table D-77 Catechism 1 Answer: Arrangement 10.0.0.0, Affectation 255.192.0.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 64 0 0
6) Abutting subnet 10 128 0 0
6) Advertisement subnet 10 192 0 0
7) Invalid subnet* 10 256 0 0
IP Acclamation Convenance 49
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Note that the subnet numbers’ absorbing octet (second octet in this case) artlessly increments by
the abracadabra number. To abate the amplitude appropriate by the table, afterwards the arrangement is obvious, the table
represents the absolute subnet numbers afore the advertisement subnet as a all-encompassing value, 10.X.0.0.
The subnets not accurately listed are 10.128.0.0, 10.160.0.0, and 10.192.0.0.
Question 3: Answer
This catechism begins with the afterward basal facts:
Network 10.0.0.0
Mask 255.248.0.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 2nd
Magic number: 256 – 248 = 8
From there, Table D-79 shows the blow of the accomplish for the process.
Table D-78 Catechism 2 Answer: Arrangement 10.0.0.0, Affectation 255.224.0.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 32 0 0
6) Abutting subnet 10 64 0 0
6) Abutting subnet 10 96 0 0
6) All-encompassing representation of abutting subnet 10 X 0 0
6) Advertisement subnet 10 224 0 0
7) Invalid subnet* 10 256 0 0
Table D-79 Catechism 3 Answer: Arrangement 10.0.0.0, Affectation 255.248.0.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 8 0 0
continues
50 CCIE Routing and Switching Exam Certification Guide
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Note that the subnet numbers’ absorbing octet (second octet in this case) artlessly increments by
the abracadabra number. To abate the amplitude appropriate by the table, afterwards the arrangement is obvious, the table
represents the absolute subnet numbers afore the advertisement subnet as a all-encompassing value, 10.x.0.0.
The subnets not accurately listed artlessly accept a assorted of 8 in the added octet.
Question 4: Answer
This catechism begins with the afterward basal facts:
Network 10.0.0.0
Mask 255.252.0.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 2nd
Magic number: 256 – 252 = 4
From there, Table D-80 shows the blow of the accomplish for the process.
Step Octet #1 Octet #2 Octet #3 Octet #4
6) Abutting subnet 10 16 0 0
6) Abutting subnet 10 24 0 0
6) All-encompassing representation of abutting subnet 10 X 0 0
6) Advertisement subnet 10 248 0 0
7) Invalid subnet* 10 256 0 0
Table D-80 Catechism 4 Answer: Arrangement 10.0.0.0, Affectation 255.252.0.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 4 0 0
6) Abutting subnet 10 8 0 0
6) Abutting subnet 10 12 0 0
Table D-79 Catechism 3 Answer: Arrangement 10.0.0.0, Affectation 255.248.0.0 (Continued)
IP Acclamation Convenance 51
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Note that the subnet numbers’ absorbing octet (second octet in this case) artlessly increments by
the abracadabra number. To abate the amplitude appropriate by the table, afterwards the arrangement is obvious, the table
represents the absolute subnet numbers afore the advertisement subnet as a all-encompassing value, 10.x.0.0.
The subnets not accurately listed artlessly accept a assorted of 4 in the added octet.
Question 5: Answer
This catechism begins with the afterward basal facts:
Network 10.0.0.0
Mask 255.255.128.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 3rd
Magic number: 256 – 128 = 128
This catechism absolutely uses a subnet acreage that spans all of the added octet, and a distinct bit in the
third octet. As a result, the aboriginal seven-step process, which assumes a 1-octet-or-less subnet
field, cannot be used. However, an advertisement action is declared forth with the acknowledgment to this
question.
First, Table D-81 shows the alpha of the process, which occurs aloof like the beforehand examples,
except that the absorbing octet is now the third octet.
Step Octet #1 Octet #2 Octet #3 Octet #4
6) All-encompassing representation of abutting subnet 10 X 0 0
6) Advertisement subnet 10 252 0 0
7) Invalid subnet* 10 256 0 0
NOTE Abounding of you may allegedly see the way to acquisition the complete acknowledgment to this question,
long afore you accomplishment account the revised action listed here. If you anticipate you are accepting the
idea, you apparently are, so do not let the capacity in the argument get in the way.
Table D-80 Catechism 4 Answer: Arrangement 10.0.0.0, Affectation 255.252.0.0 (Continued)
52 CCIE Routing and Switching Exam Certification Guide
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
At this point, the aftermost cardinal is acutely an invalid subnet cardinal due to the 256 in the third
octet. Instead of that actuality blame the end of the process, it agency you should do the following:
Record the abutting subnet, based on the afterward changes to the antecedent accurate subnet
number: add 1 to the octet to the larboard of the absorbing octet, and set the absorbing octet
to 0.
In this case, this new footfall runs as follows:
■ The antecedent accurate subnet is 10.0.128.0.
■ Add 1 to the octet to the larboard of the absorbing octet (value 0); the abutting subnet number’s second
octet will afresh be 1.
■ The abutting subnet number’s absorbing octet will be 0.
Each time the abutting subnet cardinal would accept had a 256 in the absorbing octet, you instead
follow this new step. It is a little like accustomed decimal addition. For example, back you add 319
and 1, you add 1 and 9, abode bottomward a 0, and backpack the 1 to the abutting chiffre to the left. It is much
more accessible through examples, though. So, to complete the logic, Table D-82 shows the
example, with this new argumentation implemented. (Note that the new footfall has been labeled as Footfall 8.)
Table D-81 Catechism 5 Answer, Allotment 1: Arrangement 10.0.0.0, Affectation 255.255.128.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 0 128 0
7) Invalid subnet* 10 0 256 0
Table D-82 Catechism 5 Answer, Allotment 2: Arrangement 10.0.0.0, Affectation 255.255.128.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 0 128 0
8) Increment in the octet to the left, and use 0 in
the absorbing octet
10 1 0 0
IP Acclamation Convenance 53
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
The end of the table is begin in this archetype back the octet to the larboard of the absorbing octet
reaches 256. The ahead listed subnet is the advertisement subnet.
Question 6: Answer
This catechism begins with the afterward basal facts:
Network 10.0.0.0
Mask 255.255.192.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 3rd
Magic number: 256 – 192 = 64
Like the antecedent question, this catechism absolutely uses a subnet acreage beyond than 1 octet. As a result,
the advertisement adaptation of the seven-step action is used. First, Table D-83 shows the alpha of
the process, which occurs aloof like the accepted seven-step process.
Step Octet #1 Octet #2 Octet #3 Octet #4
5) Abutting subnet 10 1 128 0
8) Increment in the octet to the left, and use 0 in
the absorbing octet
10 2 0 0
5) Abutting subnet 10 2 128 0
8) Increment in the octet to the left, and use 0 in
the absorbing octet
10 3 0 0
5) Abutting subnet 10 3 128 0
8) Increment in the octet to the left, and use 0 in
the absorbing octet
10 4 0 0
5) Abutting subnet 10 4 128 0
5) All-encompassing appearance 10 X 0/128 0
6) Advertisement subnet 10 255 128 0
7) Invalid subnet* 10 256 0 0
Table D-82 Catechism 5 Answer, Allotment 2: Arrangement 10.0.0.0, Affectation 255.255.128.0 (Continued)
54 CCIE Routing and Switching Exam Certification Guide
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
After award a 256 in the absorbing octet, the added bit of argumentation is applied, as follows:
Record the abutting subnet, based on the afterward changes to the antecedent accurate subnet
number: add 1 to the octet to the larboard of the absorbing octet, and set the absorbing octet
to 0.
Table D-84 shows the absolute values.
Table D-83 Catechism 6 Answer, Allotment 1: Arrangement 10.0.0.0, Affectation 255.255.192.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 0 64 0
5) Abutting subnet 10 0 128 0
5) Abutting subnet 10 0 192 0
7) Invalid subnet* 10 0 256 0
Table D-84 Catechism 6 Answer, Allotment 2: Arrangement 10.0.0.0, Affectation 255.255.192.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 10 0 0 0
2) Aught subnet 10 0 0 0
5) Abutting subnet 10 0 64 0
5) Abutting subnet 10 0 128 0
5) Abutting subnet 10 0 192 0
8) Increment in the octet to the left, and use 0
in the absorbing octet
10 1 0 0
5) Abutting subnet 10 1 64 0
5) Abutting subnet 10 1 128 0
5) Abutting subnet 10 1 192 0
8) Increment in the octet to the left, and use 0
in the absorbing octet
10 2 0 0
5) Abutting subnet 10 2 64 0
IP Acclamation Convenance 55
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
The end of the table is begin in this archetype back the octet to the larboard of the absorbing octet
reaches 256. The ahead listed subnet is the advertisement subnet.
Question 7: Answer
This catechism begins with the afterward basal facts:
Network 172.31.0.0
Mask 255.255.224.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 3rd
Magic number: 256 – 224 = 32
From there, Table D-85 shows the blow of the accomplish for the process.
Step Octet #1 Octet #2 Octet #3 Octet #4
5) All-encompassing appearance 10 X 0/64/128/
192
0
6) Advertisement subnet 10 255 192 0
7) Invalid subnet* 10 256 0 0
Table D-85 Catechism 7 Answer: Arrangement 172.31.0.0, Affectation 255.255.224.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 172 31 0 0
2) Aught subnet 172 31 0 0
5) Abutting subnet 172 31 32 0
5) Abutting subnet 172 31 64 0
5) Abutting subnet 172 31 96 0
5) Abutting subnet 172 31 128 0
continues
Table D-84 Catechism 6 Answer, Allotment 2: Arrangement 10.0.0.0, Affectation 255.255.192.0 (Continued)
56 CCIE Routing and Switching Exam Certification Guide
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Note that the subnet numbers’ absorbing octet (third octet in this case) artlessly increments by the
magic number.
Question 8: Answer
This catechism begins with the afterward basal facts:
Network 172.31.0.0
Mask 255.255.240.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 3rd
Magic number: 256 – 240 = 16
From there, Table D-86 shows the blow of the accomplish for the process.
Step Octet #1 Octet #2 Octet #3 Octet #4
5) Abutting subnet 172 31 160 0
5) Abutting subnet 172 31 192 0
6) Advertisement subnet 172 31 224 0
7) Invalid subnet* 172 31 256 0
Table D-86 Catechism 8 Answer: Arrangement 172.31.0.0, Affectation 255.255.240.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 172 31 0 0
2) Aught subnet 172 31 0 0
5) Abutting subnet 172 31 16 0
5) Abutting subnet 172 31 32 0
5) Abutting subnet 172 31 48 0
5) Abutting subnet 172 31 64 0
Table D-85 Catechism 7 Answer: Arrangement 172.31.0.0, Affectation 255.255.224.0 (Continued)
IP Acclamation Convenance 57
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Note that the subnet numbers’ absorbing octet (third octet in this case) artlessly increments by the
magic number. To abate the amplitude appropriate by the table, the table represents the absolute subnet
numbers afore the advertisement subnet as a all-encompassing value, 172.31.x.0. The subnets not specifically
listed artlessly accept a assorted of 16 in the third octet.
Question 9: Answer
This catechism begins with the afterward basal facts:
Network 172.31.0.0
Mask 255.255.252.0
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 3rd
Magic number: 256 – 252 = 4
From there, Table D-87 shows the blow of the accomplish for the process.
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Step Octet #1 Octet #2 Octet #3 Octet #4
5) Abutting subnet 172 31 x 0
6) Advertisement subnet 172 31 240 0
7) Invalid subnet* 172 31 256 0
Table D-87 Catechism 9 Answer: Arrangement 172.31.0.0, Affectation 255.255.252.0
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 172 31 0 0
2) Aught subnet 172 31 0 0
5) Abutting subnet 172 31 4 0
5) Abutting subnet 172 31 8 0
5) Abutting subnet 172 31 12 0
5) Abutting subnet 172 31 16 0
5) Abutting subnet 172 31 x 0
6) Advertisement subnet 172 31 252 0
7) Invalid subnet* 172 31 256 0
Table D-86 Catechism 8 Answer: Arrangement 172.31.0.0, Affectation 255.255.240.0 (Continued)
58 CCIE Routing and Switching Exam Certification Guide
Note that the subnet numbers’ absorbing octet (third octet in this case) artlessly increments by the
magic number. To abate the amplitude appropriate by the table, the table represents the absolute subnet
numbers afore the advertisement subnet as a all-encompassing value, 172.31.x.0. The subnets not specifically
listed artlessly accept a assorted of 4 in the third octet.
Question 10: Answer
This catechism begins with the afterward basal facts:
Network 172.31.0.0
Mask 255.255.255.224
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 4th
Magic number: 256 – 224 = 32
This catechism uses a subnet acreage beyond than 1 octet, acute the advertisement adaptation of the process
as apparent in Questions 5 and 6. Table D-88 shows the alpha of the process.
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Table D-88 Catechism 10 Answer, Allotment 1: Arrangement 172.31.0.0, Affectation 255.255.255.224
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 172 31 0 0
2) Aught subnet 172 31 0 0
5) Abutting subnet 172 31 0 32
5) Abutting subnet 172 31 0 64
5) Abutting subnet 172 31 0 96
5) Abutting subnet 172 31 0 128
5) Abutting subnet 172 31 0 160
5) Abutting subnet 172 31 0 192
5) Abutting subnet 172 31 0 224
7) Invalid subnet* 172 31 0 256
IP Acclamation Convenance 59
After award a 256 in the absorbing octet, the added bit of argumentation is applied, as follows:
Record the abutting subnet, based on the afterward changes to the antecedent accurate subnet
number: add 1 to the octet to the larboard of the absorbing octet, and set the absorbing octet
to 0.
Table D-89 shows the absolute values.
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
Table D-89 Catechism 10 Answer, Allotment 2: Arrangement 172.31.0.0, Affectation 255.255.255.224
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 172 31 0 0
2) Aught subnet 172 31 0 0
5) Abutting subnet 172 31 0 32
5) Abutting subnet 172 31 0 64
5) Abutting subnet 172 31 0 128
5) Abutting subnet 172 31 0 192
5) Abutting subnet 172 31 0 224
8) Increment in the octet to the left, and use 0
in the absorbing octet
172 31 1 0
5) Abutting subnet 172 31 1 32
5) Abutting subnet 172 31 1 64
5) Abutting subnet 172 31 1 128
5) Abutting subnet 172 31 1 160
5) Abutting subnet 172 31 1 192
5) Abutting subnet 172 31 1 224
8) Increment in the octet to the left, and use 0
in the absorbing octet
172 31 2 0
5) All-encompassing appearance 172 31 X Y
6) Advertisement subnet 172 31 255 224
7) Invalid subnet* 172 31 256 0
60 CCIE Routing and Switching Exam Certification Guide
The end of the table is begin in this archetype back the octet to the larboard of the absorbing octet
reaches 256. The ahead listed subnet is the advertisement subnet.
Question 11: Answer
This catechism begins with the afterward basal facts:
Network 192.168.15.0
Mask 255.255.255.192
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 4th
Magic number: 256 – 192 = 64
From there, Table D-90 shows the blow of the accomplish for the process.
Note that the subnet numbers’ absorbing octet (fourth octet in this case) artlessly increments by the
magic number.
Question 12: Answer
This catechism begins with the afterward basal facts:
Network 192.168.15.0
Mask 255.255.255.224
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 4th
Magic number: 256 – 224 = 32
Table D-90 Catechism 11 Answer: Arrangement 192.168.15.0, Affectation 255.255.255.192
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 192 168 15 0
2) Aught subnet 192 168 15 0
5) Abutting subnet 192 168 15 64
5) Abutting subnet 192 168 15 128
6) Advertisement subnet 192 168 15 192
7) Invalid subnet* 192 168 15 256
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
IP Acclamation Convenance 61
From there, Table D-91 shows the blow of the accomplish for the process.
Note that the subnet numbers’ absorbing octet (fourth octet in this case) artlessly increments by the
magic number. To abate the amplitude appropriate by the table, the table represents the absolute subnet
numbers afore the advertisement subnet as a all-encompassing value, 192.168.15.x. The subnets not specifically
listed artlessly accept a assorted of 32 in the fourth octet.
Question 13: Answer
This catechism begins with the afterward basal facts:
Network 192.168.15.0
Mask 255.255.255.240
From there, Accomplish 3 and 4 ask for the afterward pieces of information:
Interesting octet: 4th
Magic number: 256 – 240 = 16
From there, Table D-92 shows the blow of the accomplish for the process.
Table D-91 Catechism 11 Answer: Arrangement 192.168.15.0, Affectation 255.255.255.224
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 192 168 15 0
2) Aught subnet 192 168 15 0
5) Abutting subnet 192 168 15 32
5) Abutting subnet 192 168 15 64
5) Abutting subnet 192 168 15 96
5) All-encompassing appearance 192 168 15 X
6) Advertisement subnet 192 168 15 224
7) Invalid subnet* 192 168 15 256
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
62 CCIE Routing and Switching Exam Certification Guide
Note that the subnet numbers’ absorbing octet (fourth octet in this case) artlessly increments by the
magic number. To abate the amplitude appropriate by the table, the table represents the absolute subnet
numbers afore the advertisement subnet as a all-encompassing value, 192.168.15.x. The subnets not specifically
listed artlessly accept a assorted of 16 in the fourth octet.
Discovering the Aboriginal Across-the-board Summary
Route: 10 Questions
The aftermost two above sections of this addendum accommodate convenance questions to acquisition the best inclusive
and absolute arbitrary routes, respectively. For the afterward ten lists of subnets, ascertain the
subnet/mask or prefix/length for the aboriginal accessible across-the-board arbitrary route:
1. 10.20.30.0/24, 10.20.40.0/24, 10.20.35.0/24, 10.20.45.0/24
2. 10.20.7.0/24, 10.20.4.0/24, 10.20.5.0/24, 10.20.6.0/24
3. 10.20.3.0/24, 10.20.4.0/24, 10.20.5.0/24, 10.20.6.0/24, 10.20.7.0/24, 10.20.8.0/24
4. 172.16.200.0/23, 172.16.204.0/23, 172.16.208.0/23
5. 172.16.200.0/23, 172.16.204.0/23, 172.16.208.0/23, 172.16.202.0/23, 172.16.206.0/23
6. 172.16.120.0/22, 172.16.112.0/22, 172.16.124.0/22, 172.16.116.0/22
7. 192.168.1.16/29, 192.168.1.32/29, 192.168.1.24/29
Table D-92 Catechism 13 Answer: Arrangement 192.168.15.0, Affectation 255.255.255.240
Step Octet #1 Octet #2 Octet #3 Octet #4
1) Arrangement cardinal 192 168 15 0
2) Aught subnet 192 168 15 0
5) Abutting subnet 192 168 15 16
5) Abutting subnet 192 168 15 32
5) Abutting subnet 192 168 15 48
5) All-encompassing appearance 192 168 15 X
6) Advertisement subnet 192 168 15 240
7) Invalid subnet* 192 168 15 256
*The invalid subnet row is aloof a admonition acclimated by this action as to back to stop.
IP Acclamation Convenance 63
8. 192.168.1.16/29, 192.168.1.32/29
9. 10.1.80.0/25, 10.1.81.0/25, 10.1.81.128/25
10. 10.1.80.0/26, 10.1.81.0/26, 10.1.81.128/26
The afterward accomplish are a echo of the algorithm begin in Chapter 4. Chapter 4 alone explained
details bold after subnets and SLSM, but the algorithm works accomplished with SLSM or
VLSM, and with nonconsecutive subnets. However, nonconsecutive subnets about require
more passes through the algorithm logic. If VLSM is used, at Footfall 2, you decrease y from the
longest prefix breadth to alpha the process, afresh acute abounding added accomplish through the process.
Step 1 Count the cardinal of subnets; then, acquisition the aboriginal amount of y, such that
2y => that cardinal of subnets.
Step 2 For the abutting step, use a prefix breadth of the prefix breadth for anniversary of the
component subnets, bare y.
Step 3 Pretend that the everyman subnet cardinal in the account of basal subnets is
an IP address. Application the new, abate prefix from Footfall 2, account the
subnet cardinal in which this pretend abode resides.
Step 4 Echo Footfall 3 for the bigger numeric basal subnet cardinal and the
same prefix. If it is the aforementioned subnet acquired as in Footfall 3, the consistent subnet
is the best abbreviated route, application the new prefix.
Step 5 If Accomplish 3 and 4 do not crop the aforementioned consistent subnet, echo Accomplish 3 and 4,
with addition new prefix breadth of 1 beneath than the aftermost prefix length.
Question 1: Answer
This catechism begins with the afterward routes that charge to be summarized:
10.20.30.0/24
10.20.35.0/24
10.20.40.0/24
10.20.45.0/24
The aboriginal two accomplish are as follows:
1) Y = 2, because there are 4 basal routes, and 22 => 4
2) Alpha with a prefix breadth of 24 – 2 = 22
From there, Table D-93 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the two accomplish match.
64 CCIE Routing and Switching Exam Certification Guide
This catechism requires that you iterate through several progressively beneath prefix lengths until
you acquisition the actual answer. Finally, the action shows that 10.20.0.0/18 would be the smallest
inclusive summary. For questions in which the basal subnets are not consecutive, as was the
case in this question, you ability try to assumption a bigger starting point for the prefix breadth (a few bits
shorter) rather than starting with Accomplish 1 and 2 of the declared process. Regardless, the action will
give you the appropriate answer.
Question 2: Answer
This catechism begins with the afterward routes that charge to be summarized:
10.20.4.0/24
10.20.5.0/24
10.20.6.0/24
10.20.7.0/24
The aboriginal two accomplish are as follows:
1) Y = 2, because there are 4 basal routes, and 22 => 4
2) Alpha with a prefix breadth of 24 – 2 = 22
From there, Table D-94 shows the iterations through Accomplish 3 and 4. Remember, you do the math
using the aboriginal aboriginal and bigger basal subnets as if they were IP addresses, using
progressively beneath prefix lengths, until the after-effects are the same. If the after-effects are the same, then
you accept begin the aboriginal across-the-board summary.
Table D-93 Catechism 1 Answer: Across-the-board Arbitrary of 4 Routes
Prefix
Length Footfall 3 (Lowest Basal Subnet) Footfall 4 (Highest Basal Subnet)
22 10.20.30.0/22 yields a subnet of 10.20.28.0/22 10.20.45.0/22 yields a subnet of 10.20.44.0/22
21 10.20.30.0/21 yields a subnet of 10.20.24.0/21 10.20.45.0/21 yields a subnet of 10.20.40.0/21
20 10.20.30.0/20 yields a subnet of 10.20.16.0/20 10.20.45.0/20 yields a subnet of 10.20.32.0/20
19 10.20.30.0/19 yields a subnet of 10.20.0.0/19 10.20.45.0/19 yields a subnet of 10.20.32.0/19
18 10.20.30.0/18 yields a subnet of 10.20.0.0/18 10.20.45.0/18 yields a subnet of 10.20.0.0/18
Table D-94 Catechism 2 Answer: Across-the-board Arbitrary of 4 Routes
Prefix
Length Footfall 3 (Lowest Basal Subnet) Footfall 4 (Highest Basal Subnet)
22 10.20.4.0/22 yields a subnet of 10.20.4.0/22 10.20.7.0/22 yields a subnet of 10.20.4.0/22
IP Acclamation Convenance 65
Question 3: Answer
This catechism begins with the afterward routes that charge to be summarized:
10.20.3.0/24
10.20.4.0/24
10.20.5.0/24
10.20.6.0/24
10.20.7.0/24
10.20.8.0/24
The aboriginal two accomplish are as follows:
1) Y = 3, because there are 6 basal routes, and 23 => 6
2) Alpha with a prefix breadth of 24 – 3 = 21
From there, Table D-95 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
After two passes through Accomplish 3 and 4, the after-effects are equal, implying that 10.20.0.0/20 is the
smallest across-the-board summary.
Question 4: Answer
This catechism begins with the afterward routes that charge to be summarized:
172.16.200.0/23
172.16.204.0/23
172.16.208.0/23
Note that the subnets are not after in this case, but the algorithm still works. The aboriginal two
steps are as follows:
1) Y = 2, because there are 3 basal routes, and 22 => 3
2) Alpha with a prefix breadth of 23 – 2 = 21
From there, Table D-96 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
Table D-95 Catechism 3 Answer: Across-the-board Arbitrary of 6 Routes
Prefix
Length Footfall 3 (Lowest Basal Subnet) Footfall 4 (Highest Basal Subnet)
21 10.20.3.0/21 yields a subnet of 10.20.0.0/21 10.20.8.0/21 yields a subnet of 10.20.8.0/21
20 10.20.3.0/20 yields a subnet of 10.20.0.0/20 10.20.8.0/20 yields a subnet of 10.20.0.0/20
66 CCIE Routing and Switching Exam Certification Guide
After three passes through Accomplish 3 and 4, the after-effects are equal, implying that 172.16.192.0/19 is
the aboriginal across-the-board summary.
Question 5: Answer
This catechism begins with the afterward routes that charge to be summarized:
172.16.200.0/23
172.16.202.0/23
172.16.204.0/23
172.16.206.0/23
172.16.208.0/23
The aboriginal two accomplish are as follows:
1) Y = 3, because there are 5 basal routes, and 23 => 5
2) Alpha with a prefix breadth of 23 – 3 = 20
From there, Table D-97 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
Table D-96 Catechism 4 Answer: Across-the-board Arbitrary of 3 Routes
Prefix
Length
Step 3 (Lowest Component
Subnet)
Step 4 (Highest Component
Subnet)
21 172.16.200.0/21 yields a subnet of
172.16.200.0/21
172.16.208.0/21 yields a subnet of
172.16.208.0/21
20 172.16.200.0/20 yields a subnet of
172.16.192.0/20
172.16.208.0/20 yields a subnet of
172.16.208.0/20
19 172.16.200.0/19 yields a subnet of
172.16.192.0/19
172.16.208.0/19 yields a subnet of
172.16.192.0/19
Table D-97 Catechism 5 Answer: Across-the-board Arbitrary of 5 Routes
Prefix
Length Footfall 3 (Lowest Basal Subnet)
Step 4 (Highest Component
Subnet)
20 172.16.200.0/20 yields a subnet of
172.16.192.0/20
172.16.208.0/20 yields a subnet of
172.16.208.0/20
19 172.16.200.0/19 yields a subnet of
172.16.192.0/19
172.16.208.0/19 yields a subnet of
172.16.192.0/19
IP Acclamation Convenance 67
After two passes through Accomplish 3 and 4, the after-effects are equal, implying that 172.16.192.0/19 is the
smallest across-the-board summary.
Question 6: Answer
This catechism begins with the afterward routes that charge to be summarized:
172.16.112.0/22
172.16.116.0/22
172.16.120.0/22
172.16.124.0/22
The aboriginal two accomplish are as follows:
1) Y = 2, because there are 4 basal routes, and 22 => 4
2) Alpha with a prefix breadth of 22 – 2 = 20
From there, Table D-98 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
Question 7: Answer
This catechism begins with the afterward routes that charge to be summarized:
192.168.1.16/29
192.168.1.24/29
192.168.1.32/29
The aboriginal two accomplish are as follows:
1) Y = 2, because there are 3 basal routes, and 22 => 3
2) Alpha with a prefix breadth of 29 – 2 = 27
From there, Table D-99 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
Table D-98 Catechism 6 Answer: Across-the-board Arbitrary of 4 Routes
Prefix
Length Footfall 3 (Lowest Basal Subnet) Footfall 4 (Highest Basal Subnet)
20 172.16.112.0/20 yields a subnet of
172.16.112.0/20
172.16.124.0/20 yields a subnet of
172.16.112.0/20
68 CCIE Routing and Switching Exam Certification Guide
Question 8: Answer
This catechism begins with the afterward routes that charge to be summarized:
192.168.1.16/28
192.168.1.32/28
The aboriginal two accomplish are as follows:
1) Y = 1, because there are 2 basal routes, and 21 => 2
2) Alpha with a prefix breadth of 28 – 1 = 27
From there, Table D-100 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
Question 9: Answer
This catechism begins with the afterward routes that charge to be summarized:
10.1.80.0/25
10.1.81.0/25
10.1.81.128/25
The aboriginal two accomplish are as follows:
1) Y = 2, because there are 3 basal routes, and 22 => 3
2) Alpha with a prefix breadth of 25 – 2 = 23
Table D-99 Catechism 7 Answer: Across-the-board Arbitrary of 3 Routes
Prefix
Length
Step 3 (Lowest Component
Subnet)
Step 4 (Highest Component
Subnet)
27 192.168.1.16/27 yields a subnet of
192.168.1.0/27
192.168.1.32/27 yields a subnet of
192.168.1.32/27
26 192.168.1.16/26 yields a subnet of
192.168.1.0/26
192.168.1.32/26 yields a subnet of
192.168.1.0/26
Table D-100 Catechism 8 Answer: Across-the-board Arbitrary of 2 Routes
Prefix
Length Footfall 3 (Lowest Basal Subnet)
Step 4 (Highest Component
Subnet)
27 192.168.1.16/27 yields a subnet of
192.168.1.0/27
192.168.1.32/27 yields a subnet of
192.168.1.32/27
26 192.168.1.16/26 yields a subnet of
192.168.1.0/26
192.168.1.32/26 yields a subnet of
192.168.1.0/26
IP Acclamation Convenance 69
From there, Table D-101 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
Question 10: Answer
This catechism begins with the afterward routes that charge to be summarized:
10.1.80.0/26
10.1.81.0/26
10.1.81.128/26
The aboriginal two accomplish are as follows:
1) Y = 2, because there are 3 basal routes, and 22 => 3
2) Alpha with a prefix breadth of 26 – 2 = 24
From there, Table D-102 shows the iterations through Accomplish 3 and 4, application progressively shorter
prefix lengths, until the appropriate acknowledgment is found.
Discovering the Aboriginal Absolute Summary
Routes: 5 Questions
The aftermost area of this addendum provides convenance problems and answers for award exclusive
summaries. Per Chapter 4’s conventions, an absolute arbitrary may accommodate assorted prefixes/
subnets, but it may alone accommodate abode ranges central the aboriginal basal prefixes/subnets.
Table D-101 Catechism 9 Answer: Across-the-board Arbitrary of 3 Routes
Prefix Length
Step 3 (Lowest Component
Subnet)
Step 4 (Highest Component
Subnet)
23 10.1.80.0/23 yields a subnet of
10.1.80.0/23
10.1.81.128/23 yields a subnet of
10.1.80.0/23
Table D-102 Catechism 10 Answer: Across-the-board Arbitrary of 3 Routes
Prefix
Length Footfall 3 (Lowest Basal Subnet) Footfall 4 (Highest Basal Subnet)
24 10.1.80.0/24 yields a subnet of 10.1.80.0/24 10.1.81.128/24 yields a subnet of 10.1.81.0/24
23 10.1.80.0/23 yields a subnet of 10.1.80.0/23 10.1.81.128/23 yields a subnet of 10.1.80.0/23
70 CCIE Routing and Switching Exam Certification Guide
For the afterward bristles lists of subnets, ascertain the set of absolute arbitrary routes:
1. 10.20.7.0/24, 10.20.4.0/24, 10.20.5.0/24, 10.20.6.0/24
2. 10.20.3.0/24, 10.20.4.0/24, 10.20.5.0/24, 10.20.6.0/24, 10.20.7.0/24, 10.20.8.0/24
3. 172.16.200.0/23, 172.16.204.0/23, 172.16.208.0/23, 172.16.202.0/23, 172.16.206.0/23
4. 172.16.120.0/22, 172.16.112.0/22, 172.16.124.0/22, 172.16.116.0/22
5. 192.168.1.16/29, 192.168.1.32/29, 192.168.1.24/29
The afterward accomplish are a echo of the decimal algorithm for award absolute summaries found
in Chapter 4. Remember, the action assumes that all the basal subnets accept the aforementioned mask/
prefix length.
Step 1 Acquisition the best across-the-board arbitrary route; alarm it a applicant exclusive
summary route.
Step 2 Determine if the applicant arbitrary includes any abode ranges it should
not. To do so, analyze the summary’s adumbrated abode ambit with the
implied abode ranges of the basal subnets.
Step 3 If the applicant arbitrary alone includes addresses in the ranges adumbrated by
the basal subnets, the applicant arbitrary is allotment of the best exclusive
summarization of the aboriginal basal subnets.
Step 4 If instead the applicant arbitrary includes some addresses analogous the
candidate arbitrary routes, and some addresses that do not match, breach the
current applicant arbitrary in half, into two new applicant summary
routes, anniversary with a prefix 1 best than before.
Step 5 If the applicant arbitrary alone includes addresses alfresco the ranges
implied by the basal subnets, the applicant arbitrary is not allotment of the
best absolute summarization, and it should not be breach further.
Step 6 Echo Accomplish 2–4 for anniversary of the two accessible applicant arbitrary routes
created at Footfall 4.
Question 1: Answer
This catechism begins with the afterward routes that charge to be summarized:
10.20.4.0/24, ambit 10.20.4.0–10.20.4.255
10.20.5.0/24, ambit 10.20.5.0–10.20.5.255
10.20.6.0/24, ambit 10.20.6.0–10.20.6.255
10.20.7.0/24, ambit 10.20.7.0–10.20.7.255
IP Acclamation Convenance 71
The across-the-board arbitrary for these routes is
10.20.4.0/22
Table D-103 shows what turns out to be a distinct canyon through the algorithm, because the inclusive
summary and absolute arbitrary are the aforementioned for this problem.
Comparing the ambit of IP addresses in the botheration account with the ambit of addresses implied
by the aboriginal across-the-board summary, you can see that it is the exact aforementioned set of addresses. As a result,
10.20.4.0/22 is allotment of the absolute summary—in fact, no added arbitrary routes are required.
Question 2: Answer
This catechism begins with the afterward routes that charge to be summarized:
10.20.3.0/24, ambit 10.20.3.0–10.20.3.255
10.20.4.0/24, ambit 10.20.4.0–10.20.4.255
10.20.5.0/24, ambit 10.20.5.0–10.20.5.255
10.20.6.0/24, ambit 10.20.6.0–10.20.6.255
10.20.7.0/24, ambit 10.20.7.0–10.20.7.255
10.20.8.0/24, ambit 10.20.8.0–10.20.8.255
The across-the-board arbitrary for these routes is
10.20.0.0/20
Table D-104 begins by assuming three passes through the algorithm. These three passes do not determine
all the absolute arbitrary routes in the answer; Tables D-105 and D-106 complete the answer.
Before analytical Table D-104, aboriginal accede the all-embracing breeze of the afresh iterations through
the table. Anticipate of the aboriginal across-the-board arbitrary avenue as one ample accumulation of addresses. If it is
not additionally the absolute summary, you iterate through the algorithm again, halving the original
inclusive summary. If that does not aftermath an answer, you bisect anniversary of the behindhand for the next
iteration through the algorithm. So, you can anticipate of the added agreeable of the candidate
summaries as breaking them into quarters. Addition canyon would breach the aboriginal inclusive
summary into eighths, and so on. The table’s aboriginal cavalcade denotes what anniversary row agency based on
whether it is for the aboriginal across-the-board summary, the aboriginal breach (into halves), the added breach (into
quarters), and so on.
Table D-103 Catechism 1 Answer: Absolute Arbitrary of 4 Routes
Split
Candidate Exclusive
Summary
Range of
Addresses Analysis
Inclusive
summary
10.20.4.0/22 10.20.4.0–10.20.7.255 Allotment of absolute summary
72 CCIE Routing and Switching Exam Certification Guide
The aftermost four rows of the table appearance the after-effects of the added breach (per Footfall 4 in the algorithm).
Two of these four applicant absolute summaries charge to be breach afresh (10.20.0.0/22 and
10.20.8.0/22) because they accommodate some addresses aural the aboriginal ranges, but some outside
the range. One arbitrary (10.20.4.0/22) holds alone addresses central the aboriginal ranges, so that
route is one of the routes absolute the absolute summary. Finally, one applicant route
(10.20.12.0/22) contains alone addresses alfresco the aboriginal range; as a result, you can stop
splitting that ambit back attractive for the absolute summaries.
Tables D-105 and D-106 complete the official algorithm, but through some basal inspection, you
might be able to (rightfully) assumption that no added arbitrary routes will be found. Accede the
original routes, and whether the action has begin a arbitrary avenue to accommodate the addresses yet:
10.20.3.0/24—still attractive for summary
10.20.4.0/24—found summary
10.20.5.0/24—found summary
10.20.6.0/24—found summary
10.20.7.0/24—found summary
10.20.8.0/24—still attractive for summary
Table D-104 Catechism 2 Answer: Across-the-board Arbitrary of 6 Routes, Allotment 1
Split
Candidate Exclusive
Summary Ambit Analysis
Inclusive
summary
10.20.0.0/20 10.20.0.0–
10.20.15.255
Includes too abounding addresses
1st split,
lower half
10.20.0.0/21 10.20.0.0–10.20.7.255 Includes 10.20.0.0–10.20.2.255, which
should not be included
1st split,
higher half
10.20.8.0/21 10.20.8.0–
10.20.15.255
Includes 10.20.9.0–10.20.15.255, which
should not be included
2nd split,
lowest
quarter
10.20.0.0/22 10.20.0.0–10.20.3.255 Includes 10.20.0.0–10.20.2.255, which
should not be included
2nd split, 2nd
quarter
10.20.4.0/22 10.20.4.0–10.20.7.255 Includes alone 10.20.4.0–10.20.7.255; it is
part of absolute summary
2nd split, 3rd
quarter
10.20.8.0/22 10.20.8.0–
10.20.11.255
Includes 10.20.9.0–10.20.11.255, which
should not be included
2nd split,
highest
quarter
10.20.12.0/22 10.20.12.0–
10.20.15.255
Includes 10.20.12.0–10.20.15.255, totally
outside the range—don’t breach again
IP Acclamation Convenance 73
Thinking about the botheration from this point forward, the absolute basal subnets—
10.20.3.0/24 and 10.20.8.0/24—are afar by the ahead apparent 10.20.4.0/22
summary. There is alone one aboriginal avenue on anniversary ancillary of that summary. So, there is no possibility
of summarizing those two alone routes.
The algorithm will ability that aforementioned conclusion, as apparent in the abutting two tables. The third split
is in Table D-105 (Table D-104 showed up through the added split), and the fourth breach is in
Table D-106. Accumulate in apperception that, per Table D-104, alone two prefixes charge agreeable for the abutting step
in the process—10.20.0.0/22 and 10.20.8.0/22. The “Split” cavalcade in the table lists the behindhand of
these two prefixes.
(Note: Per Table D-105, alone 10.20.2.0/23 and 10.20.8.0/23 charge splitting; their behindhand are noted
in the aboriginal column.)
Table D-105 Catechism 2 Answer, 3rd Split
Split
Candidate
Exclusive
Summary Ambit Analysis
Lower bisected of
10.20.0.0/22
10.20.0.0/23 10.20.0.0–10.20.1.255 Holds none of the aboriginal addresses—don’t
split again
Higher bisected of
10.20.0.0/22
10.20.2.0/23 10.20.2.0–10.20.3.255 Includes too abounding addresses—split again
Lower bisected of
10.20.8.0/22
10.20.8.0/23 10.20.8.0–10.20.9.255 Includes too abounding addresses—split again
Higher bisected of
10.20.8.0/22
10.20.10.0/23 10.20.10.0–
10.20.11.255
Holds none of the aboriginal addresses—don’t