P-660R-Tx v2 Series User’s Guide
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APPENDIX C

IP Addresses and Subnetting

This appendix introduces IP addresses, IP address classes and subnet masks. You use subnet
masks to subdivide a network into smaller logical networks.

Introduction to IP Addresses

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.
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 255 in decim al.)
There are several classes of IP addresses. The first network number (192 in the above
example) defines the class of IP address. These are defined as follows:
• Class A: 0 to 127
• Class B: 128 to 191
• Class C: 192 to 223
• Class D: 224 to 239
• Class E: 240 to 255
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.