ZyXEL Communications ZyXEL ZyWALL 2WG - Manual (Page 663)

ZyWALL 2WG User’s Guide
663
A
P P E N D I X
E
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 00000000 to 11111111 in binary, or 0 to 255 in decimal.)
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.
The following table shows the network number and host ID arrangement for classes A, B and
C.
Table 253 Classes of IP Addresses
IP ADDRESS OCTET 1 OCTET 2 OCTET 3 OCTET 4
Class A Network number Host ID Host ID Host ID