ZyXEL Communications Internet Security Gateway ZyWALL 2 Series User Manual page 563

of ones beginning from the left most bit of the mask, followed by a continuous sequence of zeros, for a total
number of 32 bits.
Since the mask is always a continuous number of ones beginning from the left, followed by a continuous
number of zeros for the remainder of the 32 bit mask, you can simply specify the number of ones instead of
writing the value of each octet. This is usually specified by writing a "/" followed by the number of bits in
the mask after the address.
For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with mask 255.255.255.128.
The following table shows all possible subnet masks for a class "C" address using both notations.
SUBNET MASK IP ADDRESS
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
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.
IP Subnetting
Chart I-4 Alternative Subnet Mask Notation
SUBNET MASK "1" BITS
/24
/25
/26
/27
/28
/29
/30
NETWORK NUMBER
192.168.1.
11000000.10101000.00000001.
255.255.255.
11111111.11111111.11111111.
ZyWALL 2 Series User's Guide
LAST OCTET BIT
VALUE
0000 0000
1000 0000
1100 0000
1110 0000
1111 0000
1111 1000
1111 1100
HOST ID
0
00000000
0
00000000
I-3