ZyWALL 5 User’s Guide
Appendix C IP Subnetting 557
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.
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.
The first three octets of the address make up the network number (class “C”). You want to
have two separate networks.
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.
Table 15 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
Table 16 Two Subnets Example
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
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