ZyXEL Communications MWR222 - V1.0 Manual page 235

In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of
7
2 – 2 or 126 possible hosts (a host ID of all zeroes is the subnet's address itself,
all ones is the subnet's broadcast address).
192.168.1.0 with mask 255.255.255.128 is subnet A itself, and 192.168.1.127
with mask 255.255.255.128 is its broadcast address. Therefore, the lowest IP
address that can be assigned to an actual host for subnet A is 192.168.1.1 and
the highest is 192.168.1.126.
Similarly, the host ID range for subnet B is 192.168.1.129 to 192.168.1.254.
Example: Four Subnets
The previous example illustrated using a 25-bit subnet mask to divide a 24-bit
address into two subnets. Similarly, to divide a 24-bit address into four subnets,
you need to "borrow" two host ID bits to give four possible combinations (00, 01,
10 and 11). The subnet mask is 26 bits
(11111111.11111111.11111111.11000000) or 255.255.255.192.
6
Each subnet contains 6 host ID bits, giving 2 - 2 or 62 hosts for each subnet (a
host ID of all zeroes is the subnet itself, all ones is the subnet's broadcast
address).
MWR222 User's Guide
235