Subnetting
Splitting a block into equal subnets by borrowing bits, splitting into variable sizes from the largest need to the smallest, giving each subnet its network, range and broadcast, and aggregating neighbouring blocks.
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Subnet: what is it?
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A portion of an address block obtained by lengthening the prefix: the bits taken from the host part become network bits.
192.168.10.0/24split into/27gives eight subnets of 32 addresses:.0,.32,.64,.96,.128,.160,.192,.224.What is the broadcast address of
10.0.0.77/27?10.0.0.9510.0.0.12710.0.0.9610.0.0.94
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10.0.0.95A /27 has a block size of 32: 77 is in the block 64 to 95, whose broadcast is
.95and last host address.94..96opens the next block and.127ends a /25.To get at least 3 subnets out of a /24, borrow … bits, and each subnet then holds … hosts.
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To get at least 3 subnets out of a /24, borrow 2 bits, and each subnet then holds 62 hosts.
1 bit gives only 2 subnets; 2 bits give 2^2 = 4. The /24 becomes a /26: 6 host bits, 64 addresses, 62 hosts. Borrowing more bits would make smaller subnets for no reason.
Borrowing bits: what is it?
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The operation that takes k bits from the host part to number the subnets: the block yields 2^k of them, each 2^k times smaller.
For 6 subnets, 2 bits are not enough (4) and 3 bits are enough (8): borrow 3 bits, the /24 becomes /27. Always take the smallest k that works.