ZyXEL p-660h-61 Betriebsanweisung

Prestige 660H Series User’s Guide 

IP Subnetting 

       B-1 

Routers “route” based on the network number. The router that delivers the data packet to the correct 
destination host uses the host ID.  

An IP address is made up of four octets (eight bits), written in dotted decimal notation, for example, 
192.168.1.1. IP addresses are categorized into different classes. The class of an address depends on the 
value of its first octet.  

♦ 

Class “A” addresses have a 0 in the left most bit. In a class “A” address the first octet is the 
network number and the remaining three octets make up the host ID. 

♦ 

Class “B” addresses have a 1 in the left most bit and a 0 in the next left most bit. In a class “B” 
address the first two octets make up the network number and the two remaining octets make 
up the host ID. 

♦ 

Class “C” addresses begin (starting from the left) with 1 1 0. In a class “C” address the first 
three octets make up the network number and the last octet is the host ID. 

♦ 

Class “D” addresses begin with 1 1 1 0. Class “D” addresses are used for multicasting. (There 
is also a class “E” address. It is reserved for future use.)  

Class A 

0 

Network number 

Host ID 

Host ID 

Host ID 

Class B 

10 

Network number 

Network number 

Host ID 

Host ID 

Class C 

110 

Network number 

Network number 

Network number 

Host ID 

 

Host IDs of all zeros or all ones are not allowed. 

Therefore: 

♦ 

A class “C” network (8 host bits) can have 2

8

 –2 or 254 hosts.  

♦ 

A class “B” address (16 host bits) can have 2

16

 –2 or 65534 hosts.  

A class “A” address (24 host bits) can have 2

24

 –2 hosts (approximately 16 million hosts).  

Since the first octet of a class “A” IP address must contain a “0”, the first octet of a class “A” address 
can have a value of 0 to 127.  

Similarly the first octet of a class “B” must begin with “10”, therefore the first octet of a class “B” 
address has a valid range of 128 to 191. The first octet of a class “C” address begins with “110”, and 
therefore has a range of 192 to 223.