Possible Duplicate:

How does Subnetting Work?

Hi all!

I need to determine the new subnet mask showing it in dotted decimal and as a binary field, calculate the actual numbers of subnets and host in each subnet, determine if the company can subnet as desired.

*Example*:Company ABC has a class C IP address. It wants to have 5 subnets with a max of 25 computers on each subnet.

*My solution*: The default mask for the class C IP address is 255.255.255.0

There are 8 bits in the host address field: 11111111

Company ABC wants 5 subnets. The nearest possible value is 2^3 – 2 = 6.

Therefore, we take 3 top bits for the subnet field and remaining 5 bits for the host address field: ssshhhhh.

The subnet mask in our case is:
11111111 11111111 11111111 11100000

The subnet mask in dotted-decimal:
255.255.255.224

(since the corresponding bit pattern, 11100000, equates to
2^7 + 2^6+ 2^5= 128 + 64 + 32 = 224)

Actual number of subnets = 2^((number of subnet bits taken from the host) ) – 2
= 2^3– 2 = 6.

Number of hosts in each subnet = 2^((number of remaining bits in the host) ) – 2
= 2^5– 2 = 30.

Company ABC can subnet as desired.

Is it correct?