P-660HWP-Dx User’s Guide 35
APPENDIX E IP Subnetting
This appendix introduces addresses, IP address classes and subnet masks.
Introduction to IP Addresses
An IP address is made up of four octets, written in dotted decimal notation (for example,
192.168.1.1). An octet is an 8-digit binary number. Therefore, each octet has a possible range
of 00000 000 to 11111111 i n binary, o r 0 to 256 in decim al.
An IP address has two parts: the network number and the host ID. Routers use the network
number to send packets to the correct network, while the host ID identifies a single device on
the network.
IP Address Classes and Hosts
The class of an IP address determines the number of hosts you can have on your network.
• In a class A address the first octet is the network number, and the remaining three octets
are the host ID.
• In a class B address the first two octets make up the network number, and the two
remaining octets make up the host ID.
• In a class C address the first three octets make up the network number, and the last octet is
the host ID.
The following table shows the network number and host ID arrangement for classes A, B and
C.
An IP address with host IDs of all zeros is the IP address of the network. An IP address with
host IDs of all ones is the broadcast address for that network. Therefore, to determine the total
number of hosts allowed in a network, deduct two as shown next:
• A class C address (1 host octet: 8 host bits) can have 28 – 2, or 254 hosts.
• A class B address (2 host octets: 16 host bits) can have 216 – 2, or 65534 hosts.
A class A address (3 host octets: 24 host bits) can have 224 – 2 hosts, or approximately 16
million hosts.
Table 154 Classes of IP Addresses
IP ADDRESS OCTET 1 OCTET 2 OCTET 3 OCTET 4
Class A Network number Host ID Host ID Host ID
Class B Network number Network number Host ID Host ID
Class C Network number Network number Network number Host ID