ZyXEL Communications P-660H-61 User Manual page 312

Appendix B IP Addresses and Subnetting
The following table shows some possible subnet masks using both notations.
Table 106 Alternative Subnet Mask Notation
SUBNET MASK
255.255.255.0
255.255.255.128
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
8
of 2 – 2 or 254 possible hosts.
The following figure shows the company network before subnetting.
Figure 197 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.
312
ALTERNATIVE LAST OCTET
NOTATION (BINARY)
/24 0000 0000
/25 1000 0000
/26 1100 0000
/27 1110 0000
/28 1111 0000
/29 1111 1000
/30 1111 1100
LAST OCTET
(DECIMAL)
0
128
192
224
240
248
252
ADSL Router Series User's Guide