Figure 237 Subnetting Example: Before Subnetting - ZyXEL Communications IP PBX X6004 User Manual

Appendix A IP Addresses and Subnetting
Table 139 Alternative Subnet Mask Notation (continued)
SUBNET MASK
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252
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 2
The following figure shows the company network before subnetting.

Figure 237 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.
The following figure shows the company network after subnetting. There are now two sub-
networks, A and B.
290
ALTERNATIVE LAST OCTET
NOTATION (BINARY)
/26 1100 0000
/27 1110 0000
/28 1111 0000
/29 1111 1000
/30 1111 1100
8
– 2 or 254 possible hosts.
LAST OCTET
(DECIMAL)
192
224
240
248
252
X6004 User's Guide