ZyXEL NBG-318S Guia Do Utilizador

Appendix C IP Addresses and Subnetting

NBG-318S User’s Guide

Figure 131   Network Number and Host ID

How much of the IP address is the network number and how much is the host ID varies 
according to the subnet mask. 

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). The term “subnet” is short for “sub-
network”.
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. 
The following example shows a subnet mask identifying the network number (in bold text) 
and host ID of an IP address (192.168.1.2 in decimal).

Table 98   Subnet Mask - Identifying Network Number

IP Address (Binary)

11000000

10101000

00000001

00000010

Subnet Mask (Binary)

00000000

Network Number

Host ID

00000010

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.
Subnet masks can be referred to by the size of the network number part (the bits with a “1” 
value). For example, an “8-bit mask” means that the first 8 bits of the mask are ones and the 
remaining 24 bits are zeroes.