Subnetting Class B addresses in your head. Confusion!

[phreak0ut]

I find a similar question being answered in a different way. I'm not able to understand why Mr. Lammle has considered these methods.

The questions I'm presenting is from the 6th edition, Chapter 3, Page 133.

1) Question: What subnet and broadcast address is the IP address 172.16.10.33 255.255.255.224(/27) a member of?

Answer: The interesting octet is teh fourth octet. 256-224=32. 32+32=64. Bingo: 33 is between 32 and 64. However, remember that the third octet is considered part of the subnet, so the answer would be the 10.32 subnet. The broadcast is 10.63, since 10.64 is the next subnet. That was pretty easy.

Why is 32+32 done here?

2) Question: What subnet and broadcast address is the IP address 172.16.66.10 255.255.192.0(/18) a member of?

Answer:The interesting octet is the third octet instead of the fourth octet. 256-192=64. 0,64,128. The subnet is 172.16.64.0. The broadcast must be 172.16.127.255 since 128.0 is the next subnet.

Why is the valid subnet count stopped at 128 in 0,64,128?

6)Question: What is the subnet and broadcast address of the host 172.16.88.255/20?

Answer:What is a /20? If you can't answer this, you can't answer this question, can you? A /20 is 255.255.240.0, which gives us a block size of 16 in the third octet, and since no subnet bits are on in the fourth octet, the answer is always 0 and 255 in the fourth octet. 0,16,32,48,64,80,96...bingo. 88 is between 80 and 96, so subnet is 80.0 and the broadcast address is 95.255

Why is 88 considered here??

[lammle]

I really encourage you to start at the beginning of my chapter 3 and read it again. You are in the class B section answering questions that I went through with class C.

A subnet mask has a block size. This is calculated by using 256-mask.
You can easily memorize these....

for example, a 240 mask has a block size of 16. This provides us with two peices of information.
1. The subnet numbers
2. The amount of hosts in each subnet

Since, in this example, the block size was 16, our subnets we can use are 0, 16, 32, 64, 48, 80, etc...You just keep adding the block size to get your subnets.

The amount of hosts in each subnet is the block size minus 2 because of the newtork address and broadcast address of each subnet.

Please re-read this all in chapter 3.

Once you understand this, the nice part is that it is done the exact same way in class C, B and A!

cheers!
Todd Lammle

[phreak0ut]

Yes, I'm studying that chapter again. Hope I nail it soon.