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Appendix B
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 B-1 Classes of IP Addresses
IP ADDRESS:
OCTET 1
Class A
0
Network number
Class B
10
Network number
Class C
110
Network number
OCTET 2
OCTET 3
Host ID
Host ID
Network number
Host ID
Network number
Network number
Contivity 251 VPN Switch User's Guide
B-1
OCTET 4
Host ID
Host ID
Host ID
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