ZyXEL p-334wh Guia Do Utilizador

P-334WH User’s Guide

Appendix D IP Addresses and Subnetting

229

A subnet mask is used to determine which bits are part of the network number, and which bits 
are part of the host ID (using a logical AND operation). 

A subnet mask has 32 bits. If a bit in the subnet mask is a “1” then the corresponding bit in the 
IP address is part of the network number. If a bit in the subnet mask is “0” then the 
corresponding bit in the IP address is part of the host ID. 

Subnet masks are expressed in dotted decimal notation just like IP addresses. The “natural” 
masks for class A, B and C IP addresses are as follows.

Table 91    “Natural” Masks 

A

255.0.0.0

B

255.255.0.0

C

255.255.255.0

With subnetting, the class arrangement of an IP address is ignored. For example, a class C 
address no longer has to have 24 bits of network number and 8 bits of host ID. With 
subnetting, some of the host ID bits are converted into network number bits. 

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. 

Table 92   Alternative Subnet Mask Notation 

255.255.255.0

/24

0000 0000

0

255.255.255.128

/25

1000 0000

128

255.255.255.192

/26

1100 0000

192

255.255.255.224

/27

1110 0000

224