Table 230 Alternative Subnet Mask Notation; Table 231 Two Subnets Example - ZyXEL Communications ZyWall 35 User Manual

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.

Table 230 Alternative Subnet Mask Notation

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.

Table 231 Two Subnets Example

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.
Appendix C IP Subnetting
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 35 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
599