ZyXEL 660H-63 91-004-582009B Guida Utente

Prestige 660H/HW Series User’s Guide

 Splitters and Microfilters

382

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. 

Table 130   Alternative Subnet Mask Notation 

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.

As an example, you have a class “C” address 192.168.1.0 with subnet mask of 255.255.255.0. 

Table 131   Two Subnets Example 

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.