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If you want four subnets, you??™ll need to add
two bits of the host address to the subnet mask (since two binary bits are required to
produce four unique values).
There are eight bits within the fourth octet to use, so we??™ll take the top two bits as the
network subnet, and the bottom six as the host address. To create the new subnet mask,
you set all of the network bits to one. Setting the network bits to one in the fourth octet
now produces 11000000, which is a value of 192 in decimal. This makes your new subnet
mask 255.255.255.192.
When you use the entire octet as a subnet mask, assigning host IP addresses is easy. The
downside to using only part of an octet in the subnet mask is that now assigning IP
addresses becomes tricky. Only part of the fourth octet is now used as the host IP address.
Table 25.1 demonstrates how to assign addresses to hosts on the new subnets.
TABLE 25.1 Assigning Host Addresses on a Subnet
Subnet bits Host bits Decimal IP address
00 000000 to 111110 .0 to .63
01 000000 to 111110 .64 to .127
10 000000 to 111110 .


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