User Guide

Appendices Page 181
Just as one would go to the phone company for a phone number, there are controlling
bodies for IP addresses. The overall controlling body for IP addresses worldwide is
InterNIC. Businesses or individuals can request one or many IP addresses from InterNIC.
It’s a good idea to estimate the network’s future growth when requesting the class and
number of IP addresses requested.
Subnet Mask
The IP addressing system allows subnetworks or “interchanges” to be created and device
numbers or “extensions” to be established within these subnetworks. These numbers are
created using a mathematical device called a subnet mask. A subnet mask, like the IP
address, is a set of four numbers in dotted decimal notation. Subnet masks typically take
three forms:
• 255.0.0.0
• 255.255.0.0
• 255.255.255.0
The number 255 “masks” out the corresponding number of the IP address, resulting in IP
address numbers that are valid for the network. For example, an IP address of
123.45.67.89 and a subnet mask of 255.255.255.0 results in a sub network number of
123.45.67.0 and a device number of 89. The IP address numbers that are actually valid to
use are those assigned by InterNIC. Otherwise, anyone could set up IP addresses that are
duplicates of those at another company.
The subnet mask used for the network typically corresponds to the class of IP address
assigned. If the IP address is Class A, it uses a subnet mask of 255.0.0.0. Class B addresses
use a subnet mask of 255.255.0.0, and Class C IP addresses use a subnet mask of
255.255.255.0.
Default Gateway
A default gateway is like a long distance operator. Users can dial the operator to get
assistance connecting to the end party. In complex networks with many subnetworks,
gateways keep traffic from traveling between different subnetworks unless addressed to
travel there. While this helps to keep overall network traffic more manageable, it also
introduces another level of complexity.
To communicate with a device on another network, one must go through a gateway that
connects the two networks. Therefore, users must know the default gateway IP address.
If there is no gateway in the network, use an IP address of 0.0.0.0 in fields that apply to a
default gateway.
Network Address Translation (NAT)
NAT hides internal IP addresses by converting all internal host IP addresses to the IP
address of the firewall as packets are routed through the firewall. The firewall then
retransmits the data payload of the internal host from its own address using a translation
table to keep track of which sockets on the exterior interface equate to which sockets on
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