ZyXEL Communications ZyXEL Vantage VSG-1000 - Manual (Page 122)

VSG-1000 Vantage Service Gateway
P IP Subnetting
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 subnet.
Chart 9 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
Chart 10 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
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.
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
possible combinations of 00, 01, 10 and 11. The subnet mask is 26 bits (11111111.11111111.11111111.11000000)
or 255.255.255.192. Each subnet contains 6 host ID bits, giving 2
6 -2 or 62 hosts for each subnet (all 0’s is the
subnet itself, all 1’s is the broadcast address on the subnet).
Chart 11 Subnet 1
NETWORK NUMBER LAST OCTET BIT VALUE
IP Address 192.168.1. 0