Example - ZyXEL Communications Vantage CNM 2.0 User Manual

IP Address
IP Address (Binary)
Subnet Mask
Subnet Mask (Binary)
The first three octets of the address make up the network number (class "C"). You want to have two separate
networks.
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.
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
IP Address
IP Address (Binary)
Subnet Mask
Subnet Mask (Binary)
Subnet Address: 192.168.1.0
Broadcast Address: 192.168.1.127
IP Address
IP Address (Binary)
Subnet Mask
Subnet Mask (Binary)
Subnet Address: 192.168.1.128
Broadcast Address: 192.168.1.255
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
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.
Example: Four Subnets
The above example illustrated using a 25-bit subnet mask to divide a class "C" address space into two subnets.
Similarly to divide a class "C" address into four subnets, you need to "borrow" two host ID bits to give four
IP Subnetting
NETWORK NUMBER
192.168.1.
11000000.10101000.00000001.
255.255.255.
11111111.11111111.11111111.
subnet.
Subnet 1
NETWORK NUMBER
192.168.1.
11000000.10101000.00000001.
255.255.255.
11111111.11111111.11111111.
Lowest Host ID: 192.168.1.1
Highest Host ID: 192.168.1.126
Subnet 2
NETWORK NUMBER
192.168.1.
11000000.10101000.00000001.
255.255.255.
11111111.11111111.11111111.
Lowest Host ID: 192.168.1.129
Highest Host ID: 192.168.1.254
7
– 2 or 126 hosts for each subnet.
Vantage CNM 2.0
HOST ID
0
00000000
0
00000000
LAST OCTET BIT VALUE
0
00000000
128
10000000
LAST OCTET BIT VALUE
128
10000000
128
10000000
Q