ZyXEL Communications FMG3024-D10A User Manual

Appendix A IP Addresses and Subnetting

FMG3024-D10A / FMG3025-D10A Series User’s Guide

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).

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.

IP Address Network Number and Host ID Example

IP Address (Binary)

11000000

10101000

00000001

00000010

Subnet Mask (Binary)

00000000

Network Number

Host ID

00000010