ZyXEL NBG-318S Guia Do Utilizador

Appendix C IP Addresses and Subnetting

NBG-318S User’s Guide

You can use subnetting to divide one network into multiple sub-networks. In the following 
example a network administrator creates two sub-networks to isolate a group of servers from 
the rest of the company network for security reasons.
In this example, the company network address is 192.168.1.0. The first three octets of the 
address (192.168.1) are the network number, and the remaining octet is the host ID, allowing a 
maximum of 2

8

 – 2 or 254 possible hosts.

The following figure shows the company network before subnetting.  

Figure 132   Subnetting Example: Before Subnetting

You can “borrow” one of the host ID bits to divide the network 192.168.1.0 into two separate 
sub-networks. The subnet mask is now 25 bits (255.255.255.128 or /25).
The “borrowed” host ID bit can have a value of either 0 or 1, allowing two subnets; 
192.168.1.0 /25 and 192.168.1.128 /25. 
The following figure shows the company network after subnetting. There are now two sub-
networks, A and B. 

255.255.255.192

/26

1100 0000

192

255.255.255.224

/27

1110 0000

224

255.255.255.240

/28

1111 0000

240

255.255.255.248

/29

1111 1000

248

255.255.255.252

/30

1111 1100

252

Table 101   Alternative Subnet Mask Notation (continued)