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ZyAIR B-3000 User’s Guide

Appendix H IP Subnetting 198

Appendix H

IP Subnetting

IP Addressing

Routers “route” based on the network number. The router that delivers the data packet to the

correct destination host uses the host ID.

IP Classes

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.)

Table 71 Classes of IP Addresses

IP ADDRESS: OCTET 1 OCTET 2 OCTET 3 OCTET 4

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

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

Therefore:

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

–2 or 254 hosts.

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

–2 or 65534 hosts.

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

–2 hosts (approximately 16 million hosts).

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ZyXEL Communications ZyAIR Guide de l'utilisateur Page 199