Determining Address Block Size

donald7862003

having a little trouble with this portion of the MSPress Book 70-291

here is the question and answer please help me understand how they got b=6

what is the address block size i would need for 60 subnets and 120 hosts a piece?

equation n =32-(b+h)

answer

32-(6+7)=19

I know how they got 7 but how did they get 6 from 60 subnets

dynamik

I think the formulas just confuse things.

You have 32 bits that are used for networks and hosts. You borrow bits from the host bits in order to create subnets. IP addresses consist of four octets (8 bits), each of which have values of 0-255.

To find the number of hosts or subnets, just set things up in binary.

[erroneous info removed]

For these types of questions, it really helps to just memorize what bit position gives you which value.

bits--- 1-2-3--4---5--6---7---8----9----10---11

---

12--etc
value- 2-4-8-16-32-64-128-256-512-1024-2048-4096-etc

(even if you can't remember a value, just remember it's twice the value before it and half the value after it)

As you can see, 7 bits meets the requirement of 120 (remember to subtract 2, so you will get 126 usable addresses), and 6 bits meets the requirements of 60 (you typically do not need to subtract 2 here as modern equipment supports subnets of all 0s or all 1s, be sure to read the question carefully).