ZyXEL Communications P-2612HNU-F1 User Manual page 309

Appendix A IP Addresses and Subnetting
The following figure shows the company network before subnetting.
Figure 133 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.
309
P-2612HNU(L)-FxF User's Guide