Understanding IP Addressing B-5BackgroundThe IP addresses and routing configurations for the devices shown in the diagram are outlined below. Inaddition, 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 Ethernetconnection to the network while the Remote IP and Remote Sub fields describe the IP address and subnetmask of the remote router. This information is entered in the connection profile of the Netopia 4741.The Gateway field describes the router or workstation's default gateway, or where they will send their packets ifthe appropriate route is not known. The Static Route field, which is only shown on Router B, tells Router B whatpath to take to get to the network defined by Netopia 4741 B. Finally, the Usable IP Address field shows therange 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 addresseswhen configuring your network.With this configuration, both Customer Site A and B can gain Internet access through Routers A and B, with noreconfiguration of the ISP's equipment. The most important item in this configuration is the static route definedon Router B. This tells Router B what path to take to get to the network defined by Netopia 4741 B. Without thisinformation, 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 serve the same purpose. Touse RIP instead of a static route, enable Transmit RIP on Netopia 4741 A and Transmit and Receive RIP onRouter 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 subnetSuppose that your organization has a site with only 10 hosts and no plans to add any new hosts. You don’tneed a full Class C address for this site. Many ISPs offer Internet access with only a portion of a full Internetaddress.For example, you might obtain the Class C address 199.14.17.48, with the mask 255.255.255.240. From theprevious example, you can see that this gives you 14 host addresses to distribute to the hosts at your site. Ineffect, your existing network of 10 hosts is a subnet of the ISP’s network. Since the Class C address hasalready been reduced to subnets, you cannot further subnet your network without the risk of creating networkrouting problems (since you must use the mask issued by the ISP). This, however, is not a problematiclimitation for your small network.The advantages of this situation are the greater ease and lower cost of obtaining a subnet rather than a fullClass C address from an ISP.Distributing IP AddressesTo set up a connection to the Internet, you may have obtained a block of IP host addresses from an ISP. Whenconfiguring the Netopia 4741, you gave one of those addresses to its Ethernet port, leaving a number ofaddresses to distribute to computers on your network.
