ZyXEL Communications P-870H Series User Manual page 293

Appendix C IP Addresses and Subnetting
The following figure shows the company network after subnetting. There are now
two sub-networks, A and B.
Figure 193 Subnetting Example: After Subnetting
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.
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P-870H/HW Series User's Guide