MCSE : Security
4.
How to maximize the number of subnets for a given number of hosts:
Let us take a network ID of 168.8.0.0, and find the maximum number
of possible subnets and the corresponding subnet mask that can
accommodate at least 500 hosts. The steps involved are outlined
below:
I.
Find the Class of the IP address, in this case it is a class B
network. Class B network has the form N.N.H.H. Therefore, we have a
total of 16 bits (two octets) for assigning to internal networks and
hosts. The minimum number of host addresses required is 500. The
last octet corresponds to 2^8 = 256 hosts which is still less than
500 Hosts.. Therefore, you have to borrow one more bit from the
third octet to make it 256*2 = 512 Hosts. This leaves 7 bits in the
third octet for assigning subnet addresses. This is equal to 2^7=128
subnets.
II. Write the 7 bits available for subnetting in third octet in the
form 11111110 (last bit being the Host bit). The decimal equivalent
of the first seven bits is 2^7+2^6+2^5+2^4+2^3+2^2+2^1
= 128 + 64 +32 + 16 + 8 + 4 + 2 = 254.
III. Therefore, the subnet mask required is 255.255.254.0.
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