ZyXEL Communications P2000W ユーザーズマニュアル

P-2000W_V2 User’s Guide

Appendix B IP Subnetting

111

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.) 

Note: 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). 

Table 30   Classes of IP Addresses

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

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