ZyXEL Communications ZyXEL ZyWALL P1, User Manual

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ZyWALL P1 User’s Guide
284 Appendix B IP Subnetting
Note: In the following charts, shaded/bolded last octet bit values indicate host ID bits
“borrowed” to form network ID bits. The number of “borrowed” host ID bits determines
the number of subnets you can have. The remaining number of host ID bits (after
“borrowing”) determines the number of hosts you can have on each subnet.
The remaining 7 bits determine the number of hosts each subnet can have. Host IDs of all
zeros represent the subnet itself and host IDs of all ones are the broadcast address for that
subnet, so the actual number of hosts available on each subnet in the example above is 2
7
– 2
or 126 hosts for each subnet.
192.168.1.0 with mask 255.255.255.128 is the subnet itself, and 192.168.1.127 with mask
255.255.255.128 is the directed broadcast address for the first subnet. Therefore, the lowest IP
address that can be assigned to an actual host for the first subnet is 192.168.1.1 and the highest
is 192.168.1.126. Similarly the host ID range for the second subnet is 192.168.1.129 to
192.168.1.254.
Table 109 Subnet 1
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address 192.168.1. 0
IP Address (Binary) 11000000.10101000.00000001. 0 0000000
Subnet Mask 255.255.255. 128
Subnet Mask (Binary) 11111111.11111111.11111111. 1 0000000
Subnet Address: 192.168.1.0 Lowest Host ID: 192.168.1.1
Broadcast Address:
192.168.1.127
Highest Host ID: 192.168.1.126
Table 110 Subnet 2
NETWORK NUMBER LAST OCTET BIT VALUE
IP Address 192.168.1. 128
IP Address (Binary) 11000000.10101000.00000001. 1 0000000
Subnet Mask 255.255.255. 128
Subnet Mask (Binary) 11111111.11111111.11111111. 1 0000000
Subnet Address:
192.168.1.128
Lowest Host ID: 192.168.1.129
Broadcast Address:
192.168.1.255
Highest Host ID: 192.168.1.254