ZyXEL Communications ZyXEL ZyWALL 5, User Manual

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ZyWALL 5 User’s Guide
Appendix C IP Subnetting 555
A
P P E N D I X
C
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.)
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 12 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