Example: Two Subnets; Table 139 Alternative Subnet Mask Notation; Table 140 Two Subnets Example - ZyXEL Communications ZyAir G-2000 PlusV2 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 139 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 140 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.
Note: 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
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.
ZyXEL G-2000 Plus v2 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
387