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Subnetting Class B
Because a Class B network has 16 bits for host addresses, you have plenty of available bits
to play with when figuring out a subnet mask. Remember that you have to start with the
leftmost bit and work toward the right. For example, a Class B network would look like
x.y.0.0, with the default mask of 255.255.0.0. Using the default mask would give you one
network with 65,534 hosts.
The default mask in binary is 11111111.11111111.00000000.00000000. The 1s
represent the corresponding network bits in the IP address, and the 0s represent the host
bits. When you're creating a subnet mask, the leftmost bit(s) will be borrowed from the host
bits (0s will be turned into 1s) to become the subnet mask. You then use the remaining bits
that are still set to 0 for host addresses.
If you use only 1 bit to create a subnet mask, you have a mask of 255.255.128.0. If you
use 2 bits, you have a mask of 255.255.192.0, or 11111111.11111111.11000000.00000000.
As with subnetting a Class C address, you now have three parts of the IP address:
the network address, the subnet address, and the host address. You figure out the subnet
mask numbers the same way as you did with a Class C network (see the previous section,
“Calculating Values for an Eight-Subnet Class C Network”), but you'll end up with a lot
more hosts per subnet.
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