Easy Design

basic subnetting question?

i am in the cisco class and tring to figure one of these examples out

You should be able to use this to find out how many networks you will need.

To create the subnet mask, first remember that the purpose of the subnet mask is to separate the (32 bit) IP address into the network prefix and the host number.

First depict the IP address in binary. Take 200.168.90.0 and convert it to binary.

IP address: 11001000.10101000.01011010.00000000

First we determine what class of address it is:
If the first bit is 0 it is a class A address.
If the first two bits are 10 it is a class B address
If the first three bits are 110 it is a class C address
If the first four bits are 1110 it is a class D address
If the first four bits are 1111 it is a class E experimental address.
Our example is a class C address. Since we want to subnet as a Class C. the default subnet mask would be:

Subnet mask: 11111111.11111111.11111111.00000000 (255.255.255.0)

The formula for figuring out the number of ‘network’ bits in a subnet is:
2^n = number of subnets (2^n means ‘2’ to the power of ‘n’)
Since we know the number of subnets, we need to find ‘n’.
Because we want 16 subnets we want to add 4 – ‘1’ bits to the default subnet mask
(16 = 2^4) This will give us the following subnet mask:

Subnet mask 11111111.11111111.11111111.11110000 (255.255.255.240)

We also need to be able to find the range of assignable IP addresses on this subnet. To do this, we take the binary that tells us the subnet address, in this case 00000000, and compare it with the subnet mask 11110000. 200.168.90.0 and 255.255.255.240
00000000 IP address last number
11110000 Subnet address last number
The bits covered by the mask we will leave as they are. The rest of the bits we make high. So:
00000000 = 0
11110000 = 240
————-
00001111 = 15

So the first network would be 200.168.90.0 thru 200.168.90.15. The first and last IP address of each network cannot be used for nodes the first one is for network and the last one is the broadcast. So, the useable IP address for the network would be 200.168.90.1 thru 200.168.90.14. So we will have 14 IP address in each network. Let’s try it one more time:

The second network will be 200.168.90.16. So here it is;

00010000 = 16
11110000 = 240
————-
00011111 = 31

So this range of IP address will be 200.168.90.16 thru 200.168.90.31. So, the useable IP address would be 200.168.90.17 thru 200.168.90.30.
So with this setup we will have 16 subnets with 14 useable IP address on each network.

Cisco Security – Using SAFE as a Network Design Guide