ZyXEL NBG-419N Guia Do Utilizador

 Appendix B IP Addresses and Subnetting

NBG-419N 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.

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.

Subnet masks are expressed in dotted decimal notation just like IP addresses. The 
following examples show the binary and decimal notation for 8-bit, 16-bit, 24-bit 
and 29-bit subnet masks. 

The size of the network number determines the maximum number of possible 
hosts you can have on your network. The larger the number of network number 
bits, the smaller the number of remaining host ID bits. 

An IP address with host IDs of all zeros is the IP address of the network 
(192.168.1.0 with a 24-bit subnet mask, for example). An IP address with host 
IDs of all ones is the broadcast address for that network  (192.168.1.255 with a 
24-bit subnet mask, for example).

Network Number

Host ID

00000010

Table 82   Subnet Masks

8-bit mask 11111111

00000000

00000000

00000000

255.0.0.0

16-bit 
mask

11111111

11111111

00000000

00000000

255.255.0.0

24-bit 
mask

11111111

11111111

11111111

00000000

255.255.255.0

29-bit 
mask

11111111

11111111

11111111

11111000

255.255.255.24
8

Table 81   Subnet Mask - Identifying Network Number