ZyXEL Communications P-870HN-51B - V1.0 Manual page 309

Appendix C IP Addresses and Subnetting
The following figure shows the company network before subnetting.
Figure 196 Subnetting Example: Before Subnetting
You can "borrow" one of the host ID bits to divide the network 192.168.1.0 into
two separate sub-networks. The subnet mask is now 25 bits (255.255.255.128 or
/25).
The "borrowed" host ID bit can have a value of either 0 or 1, allowing two
subnets; 192.168.1.0 /25 and 192.168.1.128 /25.
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P-870HN-51b User's Guide