Case Communications 6401 Rugged ADSL Router Manual – Rev 1.5
SECTION 5 – ADVANCED CONFIGURATION Page 5.49
CIDR blocks and number of Host IDs per segment:
How can I calculate the Subnet Mask from a CIDR-type address?
Write down the number of bits that are in the CIDR notation (in Binary notation), divide them into 4
octets, and convert them to decimal notation. For example:
CIDR address: 212.43.43.33/27
Write down 27 bits as 1 (one), and the rest (5) as 0 (zero): 11111111111111111111111111100000
Divide them into 4 octets: 11111111.11111111.11111111.11100000
Convert to decimal: 255.255.255.224
Remember that class A networks are followed by a /8, class B networks are followed by a /16, and
class C networks are followed by a /24. The table below should allow a quick reference.
A network is called a ‘Superrnet, when the prefix boundary contains fewer bits than the network’s
natural (ie Classful) mask. A network is called a subnet when the prefix boundary contains more bits
than the networks natural mask
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