Netopia R2121, User Reference Manual

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Understanding IP Addressing C-5
Background
The IP Addresses and routing configurations for the devices shown in the diagram are outlined below. In
addition, each individual field and its meaning are described.
The “IP Address” and “Subnet Mask” fields define the IP Address and Subnet Mask of the device's Ethernet
connection to the network while the “Remote IP” and “Remote Sub” fields describe the IP Address and Subnet
mask of the remote router. This information is entered in the Connection Profile of the Netopia R2121.
The “Gateway” field describes the router or workstation's default gateway or, where they will send their packets
if the appropriate route is not known. The “Static Route” field, which is only shown on Router B, tells Router B
what path to take to get to the network defined by Netopia R2121 B. Finally, the “Usable IP Address” field
shows the range of IP Addresses available to the hosts of that network.
Note that the IP Addresses given in this section are for example purposes only. Do not use these addresses
when configuring your network.
With this configuration, both Customer Site A and B can gain Internet access through Routers A and B, with no
reconfiguration of the ISP's equipment. The most impor tant item in this configuration is the Static Route
defined on Router B. This tells Router B what path to take to get to the network defined by Netopia R2121 B.
Without this information, Customer Site B will be able to access Customer Site A, but not the Internet.
If it is not possible to define a Static Route on Router B, RIP could be enabled to ser ve the same purpose. To
use RIP instead of a Static Route, enable Transmit RIP on Netopia R2121 A and Transmit and Receive RIP on
Router B. This will allow the route from Customer Site B to propagate on Router B and Customer Site A.
Example: Working with a Class C subnet
Suppose that your organization has a site with only 10 hosts, and no plans to add any new hosts. You don’t
need a full Class C address for this site. Many ISPs offer Internet access with only a por tion of a full Internet
address.
For example, you may obtain the Class C address 199.14.17.48, with the mask 255.255.255.240. From the
previous example, you can see that this gives you 14 host addresses to distribute to the hosts at your site. In
effect, your existing network of 10 hosts is a subnet of the ISP’s network. Since the Class C address has
already been reduced to subnets, you cannot fur ther subnet your network without the risk of creating network
routing problems (since you must use the mask issued by the ISP). This, however, is not a problematic
limitation for your small network.
The advantages to this situation is the greater ease and lower cost of obtaining a subnet from an ISP rather
than a full Class C address.
Distributing IP addresses
To set up a connection to the Internet, you may have obtained a block of IP host addresses from an Internet
ser vice provider. When configuring the Netopia R2121, you gave one of those addresses to its Ethernet por t,
leaving a number of addresses to distribute to computers on your network.