ZyXEL NBG6515 User Guide - Page 177
Subnetting
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Appendix A IP Addresses and Subnetting Table 84 Alternative Subnet Mask Notation (continued) SUBNET MASK ALTERNATIVE LAST OCTET LAST OCTET NOTATION (BINARY) (DECIMAL) 255.255.255.224 /27 1110 0000 224 255.255.255.240 /28 1111 0000 240 255.255.255.248 /29 1111 1000 248 255.255.255.252 /30 1111 1100 252 Subnetting You can use subnetting to divide one network into multiple sub-networks. In the following example a network administrator creates two sub-networks to isolate a group of servers from the rest of the company network for security reasons. In this example, the company network address is 192.168.1.0. The first three octets of the address (192.168.1) are the network number, and the remaining octet is the host ID, allowing a maximum of 28 - 2 or 254 possible hosts. The following figure shows the company network before subnetting. Figure 131 Subnetting Example: Before Subnetting You can "borrow" one of the host ID bits to divide the network 192.168.1.0 into two separate subnetworks. 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. The following figure shows the company network after subnetting. There are now two subnetworks, A and B. NBG6515 User's Guide 177