ZyXEL Communications 10 Series, Reference Manual

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ZyWALL 10~100 Series Internet Security Gateway
IP Subnetting 8-1
Chapter 8
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.)
Chart 8-1 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
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.