Asus GigaX3112 - Manual (Page 104)

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GigaX3112 Series Layer 3 Managed Switch
Note
For example, consider a class C network 192.168.1. To split this into two
subnets, you would use the subnet mask:
255.255.255.128
Itʼs easier to see whatʼs happening if we write this in binary:
11111111. 11111111. 11111111.10000000
As with any class C address, all of the bits in field1 through field 3 are part of
the network ID, but note how the mask specifies that the first bit in field 4 is also
included. Since this extra bit has only two values (0 and 1), this means there
are two subnets. Each subnet uses the remaining 7 bits in field4 for its host IDs,
which range from 0 to 127 (instead of the usual 0 to 255 for a class C address).
Similarly, to split a class C network into four subnets, the mask is:
255.255.255.192 or 11111111. 11111111. 11111111.11000000
The two extra bits in Field 4 can have four values (00, 01, 10, 11), so there are
four subnets. Each subnet uses the remaining six bits in field4 for its host IDs,
ranging from 0 to 63.
Sometimes a subnet mask does not specify any additional
network ID bits, thus no subnets. Such a mask is called a default
subnet mask. These masks are:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
These are called default because they are used when a network is initially
configured, at which time it has no subnets.