ZyXEL Communications ZyAIR G-405, User Manual

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ZyAIR G-405 User’s Guide
IP Subnetting M
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 8-4 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.
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.