ZyXEL Communications Internet Security Gateway ZyWALL 2 Series User Manual page 561

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.)
IP ADDRESS:
Class A 0
Class B 10
Class C 110
Therefore:
IP Subnetting
Chart I-1 Classes of IP Addresses
OCTET 1
Network number
Network number
Network number
Host IDs of all zeros or all ones are not allowed.
A class "C" network (8 host bits) can have 2
A class "B" address (16 host bits) can have 2
OCTET 2
Host ID
Network number
Network number
ZyWALL 2 Series User's Guide
Appendix I
IP Subnetting
OCTET 3
Host ID
Host ID
Network number
8
–2 or 254 hosts.
16
–2 or 65534 hosts.
OCTET 4
Host ID
Host ID
Host ID
I-1