ZyXEL Communications ZYWALL 2 WG, User Manual

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ZyWALL 2WG User’s Guide
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P P E N D I X
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IP Addresses and Subnetting
This appendix introduces IP addresses and subnet masks.
IP addresses identify individual devices on a network. Every networking device (including
computers, servers, routers, printers, etc.) needs an IP address to communicate across the
network. These networking devices are also known as hosts.
Subnet masks determine the maximum number of possible hosts on a network. You can also
use subnet masks to divide one network into multiple sub-networks.
Introduction to IP Addresses
One part of the IP address is the network number, and the other part is the host ID. In the same
way that houses on a street share a common street name, the hosts on a network share a
common network number. Similarly, as each house has its own house number, each host on the
network has its own unique identifying number - the host ID. Routers use the network number
to send packets to the correct network, while the host ID determines to which host on the
network the packets are delivered.
Structure
An IP address is made up of four parts, written in dotted decimal notation (for example,
192.168.1.1). Each of these four parts is known as an octet. An octet is an eight-digit binary
number (for example 11000000, which is 192 in decimal notation).
Therefore, each octet has a possible range of 00000000 to 11111111 in binary, or 0 to 255 in
decimal.
The following figure shows an example IP address in which the first three octets (192.168.1)
are the network number, and the fourth octet (16) is the host ID.