ZyXEL Communications ZyWALL 5 User Manual page 497

SUBNET MASK IP ADDRESS
255.255.255.248
255.255.255.252
The first mask shown is the class "C" natural mask. Normally if no mask is specified it is understood
that the natural mask is being used.
Example: Two Subnets
As an example, you have a class "C" address 192.168.1.0 with subnet mask of 255.255.255.0.
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 subnet.
IP Address
IP Address (Binary)
Subnet Mask
Subnet Mask (Binary)
Subnet Address: 192.168.1.0
Broadcast Address: 192.168.1.127
IP Subnetting
Chart J-4 Alternative Subnet Mask Notation
SUBNET MASK "1" BITS
/29
/30
NETWORK NUMBER
192.168.1.
11000000.10101000.00000001.
255.255.255.
11111111.11111111.11111111.
Chart J-5 Subnet 1
NETWORK NUMBER
192.168.1.
11000000.10101000.00000001.
255.255.255.
11111111.11111111.11111111.
ZyWALL 5 Internet Security Appliance
LAST OCTET BIT VALUE
1111 1000
1111 1100
0
00000000
0
00000000
LAST OCTET BIT VALUE
0
00000000
128
10000000
Lowest Host ID: 192.168.1.1
Highest Host ID: 192.168.1.126
HOST ID
J-3