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Consider routing table of an organization’s router shown below: Subnet Number  Subnet Mask Next Hop 12.20.164.0 255.255.252.0 R1 12.20.170.0 255.255.254.0 R2 12.20.168.0  255.255.254.0  Interface 0 12.20.166.0 255.255.254.0 Interface 1  default   R3 Which of the following prefixes in CIDR notation can be collectively used to correctly aggregate all of the subnets in the routing table?

Consider routing table of an organization’s router shown below: Subnet Number  Subnet Mask Next Hop 12.20.164.0 255.255.252.0 R1 12.20.170.0 255.255.254.0 R2 12.20.168.0  255.255.254.0  Interface 0 12.20.166.0 255.255.254.0 Interface 1  default   R3 Which of the following prefixes in CIDR notation can be collectively used to correctly aggregate all of the subnets in the routing table? Correct Answer <span style="">12.20.164.0/20</span>, <span style="">12.20.164.0/21</span>

Concept:

CIDR notation:

CIDR notation (Classless Inter-Domain Routing) is an alternate method of representing a subnet mask. It is simply a count of the number of network bits (bits that are set to 1) in the subnet mask.

Explanation:

IP prefixes can be aggregated, by merging specific, long prefixes into broader, smaller ones. If filtering is done, we may delete those specific prefixes from the forwarding table and save state information. This is a way to improve routing scalability.

1. 

12.20.164.0  

12.20.10100100.00000000

255.255.252.0

255.255.11111100.00000000
   2.    

12.20.170.0 

12.20.10101010.00000000

255.255.254.0

255.255.11111110.00000000
3.

12.20.168.0 

12.20.10101000.00000000

255.255.254.0

255.255.11111110.00000000
 4. 

12.20.166.0 

12.20.10100110.00000000

255.255.254.0

255.255.11111110.00000000

Subnet (2) and (3) can be aggregated.

12.20.101010  10.00000000

12.20.101010  00.00000000

----------------------------------------------------

12.20.101010   00.00000000  = 12.20.168.0/22

Subnet (1) and (4) can be aggregated.

12.20.101001  00.00000000

12.20.101001  10.00000000

----------------------------------------------

12.20.101001  10 .00000000   =  12.20.134.0/22

Hence the correct answer is option 2, option 4.

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