ZyXEL NBG-318S Guia Do Utilizador

 Appendix C IP Addresses and Subnetting

NBG-318S User’s Guide

Figure 133   Subnetting Example: After Subnetting

In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of 2

7

 – 2 or 126 

possible hosts (a host ID of all zeroes is the subnet’s address itself, all ones is the subnet’s 
broadcast address).
192.168.1.0 with mask 255.255.255.128 is subnet A itself, and 192.168.1.127 with mask 
255.255.255.128 is its broadcast address. Therefore, the lowest IP address that can be assigned 
to an actual host for subnet A is 192.168.1.1 and the highest is 192.168.1.126. 
Similarly, the host ID range for subnet B is 192.168.1.129 to 192.168.1.254.

The previous example illustrated using a 25-bit subnet mask to divide a 24-bit address into two 
subnets. Similarly, to divide a 24-bit address into four subnets, you need to “borrow” two host 
ID bits to give four possible combinations (00, 01, 10 and 11). The subnet mask is 26 bits 
(11111111.11111111.11111111.11000000) or 255.255.255.192. 
Each subnet contains 6 host ID bits, giving 2

6

 - 2 or 62 hosts for each subnet (a host ID of all 

zeroes is the subnet itself, all ones is the subnet’s broadcast address). 

Table 102   Subnet 1 

IP Address (Decimal)

192.168.1.

0

IP Address (Binary)

11000000.10101000.00000001. 

00000000

Subnet Mask (Binary)

11111111.11111111.11111111. 

11000000

Subnet Address: 

192.168.1.0

Lowest Host ID: 192.168.1.1

Broadcast Address: 

192.168.1.63

Highest Host ID: 192.168.1.62