Example: two subnets – ZyXEL Communications 802.11g HomePlug AV ADSL2+ Gateway P-660HWP-Dx User Manual

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Appendix E IP Subnetting

P-660HWP-Dx User’s Guide

By convention, subnet masks always consist of a continuous sequence of ones beginning from
the leftmost 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.

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”).
To make two 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 157 Alternative Subnet Mask Notation

SUBNET MASK

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 158 Two Subnets Example

IP/SUBNET MASK

NETWORK NUMBER

HOST ID

IP Address

192.168.1.

IP Address (Binary)

11000000.10101000.00000001.

00000000

Subnet Mask

255.255.255.

Subnet Mask (Binary)

11111111.11111111.11111111.

00000000