Subneting question

wizarddeath

Ok, so Im trying to hone my subnet skills, I can get my head around most if I get them wrong, but looking for some help here.

The question:

Question: You are designing a subnet mask for the 10.0.0.0 network. You want 3000 subnets with up to 3400 hosts on each subnet. What subnet mask should you use?

Answer: 255.255.240.0


My answer was /21
255.255.248 or 11111111.11111111.11111000.00000000

Because 13 network bytes barrowed = 4096 subnets which was the closest to 3k subnets, with more then enough remaining host. Help!

dmaftei

With /21 (mask 255.255.248.0) you get 8192 subnets, with 2048 addresses (i.e. 2046 hosts) per subnet.

With /20 (mask 255.255.240.0) you get 4096 subnets, with 4096 addresses (i.e. 4094 hosts) per subnet.

wizarddeath

Grrr stuck in the octet mode, thinking 128 was 8 and not 7..... I see now, thanks.

evanderburg

In other words, 12 bits borrowed is 4096. 12 both ways.

2^10=1024
2^11=2048
2^12=4096

nuglobe

For the CCNA should I know my powers that high like the back of my hand?

Humper

It's really not hard, I remember specifics in the upper range like 2^10 was 1024, so I knew if I needed to know 2^11 then that would be 1024 doubled so 2048....2^12 is 4096...

I would focus mostly on class B and C for the exam.

bernie1442

nuglobe
PostPosted: Tue Jun 20, 2006 10:39 pm Post subject:
For the CCNA should I know my powers that high like the back of my hand?

A trick that I learned that may be usefull to others is just to memorize the 1st 10 powers of 2, this will let you make a failrly close estimate of the higher powers. For example 2^4 = 16.....2^14 = 16,384.