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**bilbus** · 02-12-2009, 12:06 AM

from a ccna question

By borrowing 5 bits from the last octet, you can. have 30 subnets. If you borrowed only 4 bits you could only have 14 subnets. The formula is (2 to the power of n)-2. By borrowing 4 bits, you have (2×2x2×2)-2=14. By borrowing 5 bits, you have (2×2x2×2x2)-2=30. To get 20 subnets, you would need to borrow 5 bits so the subnet mask would be 255.255.255.248. You are given the IP address of 172.16.2.160 with a subnet mask of 255.255.0.0. What is the network address in binary?
* 10101100 00010000
* 00000010 10100000
* 10101100 00000000
* 11100000 11110000
Correct answer: A

02-12-2009, 12:06 AM	#2 (permalink)
bilbus TSF Enthusiast Join Date: Aug 2006 Posts: 949 OS: OS2 Warp	Re: Subnetting Doubt from a ccna question By borrowing 5 bits from the last octet, you can. have 30 subnets. If you borrowed only 4 bits you could only have 14 subnets. The formula is (2 to the power of n)-2. By borrowing 4 bits, you have (2×2x2×2)-2=14. By borrowing 5 bits, you have (2×2x2×2x2)-2=30. To get 20 subnets, you would need to borrow 5 bits so the subnet mask would be 255.255.255.248. You are given the IP address of 172.16.2.160 with a subnet mask of 255.255.0.0. What is the network address in binary? * 10101100 00010000 * 00000010 10100000 * 10101100 00000000 * 11100000 11110000 Correct answer: A