CCNA - Subnet Question

Author

Subnet Question

dygitaljoe

2003-09-10, 10:28 pm

Ok, recently failed test and have a question there which is kinda hard.

With the IP network 176.16.8.0 need to split up subnets to have 468 hosts/net and fill max subnet space. What is the highest (or lowest) subnet mask?

I modified the question a little from test. But I want to know how to answer one of these fairly quickly during exam.

BTW, in my test they extended the time from 75 minutes to 90, but this isnt TOO much of a time extension, still felt rushed.

Lamont

2003-09-11, 12:47 am

I passed the test today and had a very similar question to this one. To figure these out quickly during the test, you should either memorize or be able to quickly calculate your "powers of two" all the way up to 2^12.

Remember that the number of hosts on a subnet is 2^N -2, where N is the number of unmasked bits. So if you need 468 hosts you need to determine the power of two that will be larger than the number of hosts you need. In this case 2^9=512 & 512-2=510 which is more than the number of hosts required. (2^8=256 which is not enough hosts).

Now you know how many unmasked bits you need, so you should be able to figure out easily how many masked bits you need and figure out your subnet mask (in this case 255.255.254.0)

Humbug

2003-09-11, 2:38 am

Very easy,

First, collect the facts :

1. Netowrk : 176.16.8.0 (Class B)
2. Mask : 255.255.0.0

Second, understand what you need to do :

1. Create subnet with 468 Hosts .

Now, 2 power what = 468 ?
Answer : 2^9 = 512.

Original Mask :
255.255.0.0

Why need to at least 9 bits for the host id :

255.255.11111110.00000000

Or

255.255.254.0

And how many subnets we will have ?

(16-9)^2 = 128 - 2 = 126 subnets .

dygitaljoe

2003-09-16, 11:52 am

Thanks for your help very much.

I am also studying Todd Lammle's way of subnetting, the five questions.... not to bad, and memorized powers of two:

2^2= 4
2^3= 8
2^4= 16
2^5= 32
2^6= 64
2^7= 128
2^8= 256
2^9= 512
2^10= 1024
2^11= 2048
2^12= 4096

Yay!

NetStroller

2003-09-16, 2:36 pm

Here's one that helped me a lot.

http://www.mcsefreak.com/subnetting.htm

With subnetting, practice makes perfect. I had problems with it at first, but now can do lot of them in my head, especially with the chart in the above tutorial.

Don't skip over hard subnet problems, because working through them will help you understanding them, which will be very helpful in figuring out subnetting problems quickly.

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