ZyXEL Communications eir F1000, User Manual

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310
eir F1000 Modem U
ser’s Guide
Appendix B IP Addresses and Subnetting
The following table shows some possible subnet masks using both notations.
Table 131
Alternative Subnet Mask Notation
SUBNET MASK
ALTERNATIVE
NOTATION LAST OCTET
(BINARY) LAST OCTET
(DECIMAL)
255.255.255.0 /24 0000 0000 0
255.255.255.128 /25 1000 0000 128
255.255.255.192 /26 1100 0000 192
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 2
8
– 2 or 254 possible hosts.
The following figure shows the company network before subnetting.
Figure 206
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.