ZyXEL Communications ADSL2+ Ethernet/USB Gateway 660RU-Tx Series - Manual (Page 264)

Prestige 660RU-Tx Series User’s Guide
263 Appendix C
Divide the network 192.168.1.0 into two separate subnets by converting one of the host ID bits
of the IP address to a network number bit. The “borrowed” host ID bit can be either “0” or “1”
thus giving two subnets; 192.168.1.0 with mask 255.255.255.128 and 192.168.1.128 with
mask 255.255.255.128.
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.
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.
Table 88 Subnet 1
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address 192.168.1. 0
IP Address (Binary) 11000000.10101000.00000001. 00000000
Subnet Mask 255.255.255. 128
Subnet Mask (Binary) 11111111.11111111.11111111. 10000000
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 89 Subnet 2
NETWORK NUMBER LAST OCTET BIT VALUE
IP Address 192.168.1. 128
IP Address (Binary) 11000000.10101000.00000001. 10000000
Subnet Mask 255.255.255. 128
Subnet Mask (Binary) 11111111.11111111.11111111. 10000000
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