IP Subnet Masks, Notation and Subnet Calculations
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Subnet Mask Notation
Like IP addresses, subnet masks are always used as a 32-bit binary number by computers. And like IP addresses, using them as 32-bit binary numbers is difficult for humans. Therefore, they are usually converted to dotted decimal notation for convenience, just like IP addresses are.
Let's take a quick example to show what this is all about. Suppose we have the Class B network 220.127.116.11. We decide to subnet this using 5 bits for the subnet ID and 11 bits for the host ID. In this case, the subnet mask will have 16 ones for the network portion (since this is Class B) followed by 5 ones for the subnet ID, and 11 zeroes for the host ID. That's 11111111 11111111 11111000 00000000 in binary, with the bits corresponding to the subnet ID highlighted. Converting to dotted decimal, the subnet mask would be 255.255.248.0. Figure 66 illustrates this process.
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