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Monday, September 5, 2011

Which Subnet Mask gives you 64 Sub-networks from a class B network address?

In other words you want to take one network address and create 64 sub-networks from it!
To create sub-networks, you take bits from the host portion of the IP address and reserve them to define the subnet address.

Default subnet mask for all class B addresses is 255.255.0.0 or 11111111.11111111.00000000.00000000 or /16 where the “1”s define the network portion and “0”s define the host portion.

Number of subnets = 2^n
Where n is the number of “1”s
ð  64= 2^n
ð  2^6 = 2^n
ð  6 = n                                                                                                                                                                                                               
Which means we shall take 6 “0”s from the host portion and convert them to “1”s; The new subnet mask will be 11111111.11111111.11111100.00000000 or 255.255.252.0 or /22
Answer = 255.255.252.0 or /22
Leaving only 10 “0”s to define the hosts;
Number of Hosts = (2^m) - 2
Where m = number of “0”s
ð  Number of Hosts = (2^10) -2
ð  Number of Hosts = 1024 -2 =1022
You need to subtract 2 for the subnet address and the broadcast address, which are not valid hosts.
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