A Two Segment Network Connected to an ISP via an Ascend Pipeline ISDN Router

Another common configuration is for a network to have two segments, one running on standard Ethernet at 10 megabits per second, and another one running on Fast Ethernet at 100 megabits per second. In this example the two networks are connected through a single server system that can route IP, such as a Microsoft Windows NT system or a UNIX server. This Router system is configured with two Ethernet Network Interface Cards (NICs), one on each network segment. This entire network is connected to an ISP by a permanent ISDN connection via an Ascend Pipeline ISDN Router. The ISP assigns the class C network 205.217.146.0 to the customer and requests that the user have the ISDN router dial in using the IP address 205.217.146.1. In this case the class C network is subnetted using the subnet mask 255.255.255.192, yielding two subnets: 205.217.146.64 and 205.217.146.128, each capable of hosting 62 systems. The first is allocated for the 10 megabit network, which is connected to the ISDN router, and the second is connected to the 100 megabit network. To the ISP the client network looks like a single class C network, but internally it is routed as two subnets. The ISDN router is configured with an IP address of 205.217.146.1/24. This causes it to dial into the ISP using this address. The ISP will route the entire 254 addresses of the class C network across this connection to the router. The Ethernet connection of the ISDN router is configured with an address of 205.217.146.65/26. This is the lowest host address of the 10 megabit network with a netmask of 255.255.255.192. A Static Route is added to the ISDN router with a destination of 205.217.146.128/26 and a Gateway of 205.217.146.127. This route will forward packets that have a destination on the 100 megabit network to the 10 megabit interface of the Router system. The Router system that has an interface on both networks is set up as follows: The interface on the 10 megabit network is given the IP address 205.217.146.127 with a netmask of 255.255.255.192. The interface on the 100 megabit network is given an IP address of 205.217.146.129 with a netmask of 255.255.255.192. The default gateway on the system is set to 205.217.146.65, the address of the Ethernet port of the ISDN router. This system must have IP forwarding enabled for it to pass packets from one network to the other. All other systems on the 10 megabit network are given addresses in the range 205.217.146.66 through 205.217.146.126, with a netmask of 255.255.255.192. They all have their default gateway set to 205.217.146.65, the address of the Ethernet port of the ISDN router. Any systems on the 10 megabit network that would be doing heavy communication with systems on the 100 megabit network could have a static route added to their routing table. The route would be configured with a destination of 205.217.146.128, a netmask of 255.255.255.192, and a gateway of 205.217.146.127. This would send packets with destinations on the 100 megabit side directly to the Router system. Otherwise packets from systems on the 10 megabit segment that have destinations on the 100 megabit network would go to the ISDN router. The packets would then be sent back across the same segment to the Router system for delivery. All other systems on the 100 megabit network are given addresses in the range 205.217.146.130 through 205.217.146.191 with a netmask of 255.255.255.192. They all have their default gateway set to 205.217.146.129, the address of the 100 megabit side of the Router system that is on both networks.