Asus GigaX 2008EX User Manual page 62

GigaX2008EX L2 Managed Switch User Manual
more bits from the host ID portion of the address. The subnet mask identifies
these host ID bits.
For example, consider a class C network 192.168.1. To split this into two
subnets, you would use the subnet mask:
Itʼs easier to see whatʼs happening if we write this in binary:
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.
These are called default because they are used when a network is initially
configured, at which time it has no subnets.
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255.255.255.128
11111111. 11111111. 11111111.10000000
Sometimes a subnet mask does not specify any additional network
ID bits, and 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