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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 the interior interface. To the
Internet, all of the traffic on the network appears to come from the same computer.
Performing the Initial Configuration Page 37
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