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Trainee

Joined: Tue Nov 29, 2011 11:57 pm
Posts: 1
Hello everyone and a special warm thanks to professor Messor your teaching style is amazing.

Here is my question

From the videos i understand that in order to get the network address (Subnet ID) you would take the mask and IP address and convert to binary then use the (bitwise and). Then take all the zeros and make them ones to get the broadcast address. When applying it to the following address i cant seem to get it to work.

200.10.5.68/28

I get the following
Subnet ID 200.10.5.64

What am i doing wrong. Thanks in advance

Wed Nov 30, 2011 12:13 am

Joined: Fri Apr 27, 2007 10:58 pm
Posts: 475
Let's step through the process:

If you do the calculations of the 28 bits, it comes out to a decimal representation of the subnet mask 255.255.255.240.

I've attached the screen shot of the calculation.

The important consideration is that last byte of the address and subnet mask. Because you are masking the first four bits of the last octet, you don't count the last four bits when you calculate your subnet address. Therefore, the last number of your network address is 64 decimal.

To get the broadcast address for this range, you change the last four un-masked bits to ones and calculate the entire byte. This means the broadcast address is 79.

Attachments:

subnet-slide.015.jpg [ 71.67 KiB | Viewed 471 times ]
Wed Nov 30, 2011 1:05 am
Expert Member

Joined: Fri Feb 11, 2011 1:31 pm
Posts: 145
jfamy81 wrote:
Hello everyone and a special warm thanks to professor Messor your teaching style is amazing.

Here is my question

From the videos i understand that in order to get the network address (Subnet ID) you would take the mask and IP address and convert to binary then use the (bitwise and). Then take all the zeros and make them ones to get the broadcast address. When applying it to the following address i cant seem to get it to work.

200.10.5.68/28

I get the following
Subnet ID 200.10.5.64

What am i doing wrong. Thanks in advance

I do this differently but I don't use the bitwise method. I assume that's the ANDing process?
with an ip of 200.10.5.68/28 the subnet mask would be 255.255.255.240 and the network the host belongs to is 200.10.5.64. Broadcast is 200.10.5.79 which is the last IP before it hits .80
Host range is 200.10.5.69 to 200.10.5.78
The method I use allows me to do subnetting in my head and usually within a few seconds. I'm not trying to boast(far from it) but just saying there's much better ways(easier) to subnet and ensure you get it right every time. I have noticed there's many different ways and some ways can actually make subnetting harder than it actually is. I honestly don't see the point of adding up loads of binary ones and zeroes as surely this makes you more prone to error.

a good website to practice subnetting is http://faculty.valleycollege.net/rpowel ... ubnet2.htm

but there's others out there.You may already have tried this.

_________________
A+,Net+Sec+,CCENT,CCNA,CWTS, CCNP

Wed Nov 30, 2011 4:03 pm

Joined: Fri Apr 27, 2007 10:58 pm
Posts: 475
fizzydizzy wrote:
The method I use allows me to do subnetting in my head and usually within a few seconds. I'm not trying to boast(far from it) but just saying there's much better ways(easier) to subnet and ensure you get it right every time. I have noticed there's many different ways and some ways can actually make subnetting harder than it actually is. I honestly don't see the point of adding up loads of binary ones and zeroes as surely this makes you more prone to error.

The point of stepping through the actual binary calculation is that it's less prone to errors. The calculation is the same every time, and you can be assured that you have the right answer.

Unfortunately, shortcuts sometimes miss a few important steps. The answers you provided, for example, weren't entirely correct.

You find your own shortcuts as you do more subnetting, but I think it's important to know the fundamentals behind the calculations before you streamline the process.

Wed Nov 30, 2011 9:39 pm
Expert Member

Joined: Fri Feb 11, 2011 1:31 pm
Posts: 145
Yes i did make an error on the host range, should have been 200.10.5.65 not .69. Simple mistype and one I should have rechecked. But for the exam as I have discovered its best to use the increment method of subnetting assuming a person knows the basics as you said.
My point was that adding up binary can be more prone to errors because we are staring at loads of ones and zeros and are clock watching at same time. By all means practice with it in the comfort of your own house but trying this is the exam with either a Cisco or Comptia related subnetting question may eat up a few minutes.

_________________
A+,Net+Sec+,CCENT,CCNA,CWTS, CCNP

Fri Dec 02, 2011 2:14 pm
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