ZyXEL Communications 645R-A Series User Manual
Prestige 645R-A Series User’s Guide
IP Subnetting
B-3
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.
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.
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.
Chart B-4 Alternative Subnet Mask Notation
SUBNET MASK IP ADDRESS
SUBNET MASK “1” BITS
LAST OCTET BIT VALUE
255.255.255.0 /24
0000
0000
255.255.255.128 /25
1000
0000
255.255.255.192 /26
1100
0000
255.255.255.224 /27
1110
0000
255.255.255.240 /28
1111
0000
255.255.255.248 /29
1111
1000
255.255.255.252 /30
1111
1100
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.
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.
NETWORK NUMBER
HOST ID
IP Address
192.168.1.
0
IP Address (Binary)
11000000.10101000.00000001. 00000000
Subnet Mask
255.255.255.
0
Subnet Mask (Binary)
11111111.11111111.11111111. 00000000
The first three octets of the address make up the network number (class “C”). You want to have two separate
networks.
networks.