Appendix Jip Subnetting - ZyXEL Communications ZyWALL 5 User Manual

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 Network number
Class B 10 Network number
Class C 110 Network number
Host IDs of all zeros or all ones are not allowed.
Therefore:
A class "C" network (8 host bits) can have 2
A class "B" address (16 host bits) can have 2
A class "A" address (24 host bits) can have 2
Since the first octet of a class "A" IP address must contain a "0", the first octet of a class "A" address
can have a value of 0 to 127.
Similarly the first octet of a class "B" must begin with "10", therefore the first octet of a class "B"
address has a valid range of 128 to 191. The first octet of a class "C" address begins with "110", and
therefore has a range of 192 to 223.
IP Subnetting
Chart J-1 Classes of IP Addresses
OCTET 1 OCTET 2
Host ID
Network number
Network number
24
–2 hosts (approximately 16 million hosts).
ZyWALL 5 Internet Security Appliance
Appendix J
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
J-1