ZyXEL Communications Prestige 645R-A Series, User Manual, Page: 221 / 251

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.
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.
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.
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.