A Class C Network Connected to an ISP by a Cisco 2501 Router

One extremely common network configuration is a network connected to an ISP by a T1 line and a Cisco 2501 router. The ISP routes a complete class C network across the T1 to the router. This configuration can serve a single network segment with up to 253 individual hosts or virtual servers. In this example the class C network address assigned to the customer by the ISP is 205.217.146.0. The Ethernet port on the router is given the IP address 205.217.146.1. It is not required, but it is common in this type of class C network for the router to be given the host number 1 or 254. Serial port 0 of the router is connected to a CSU/DSU, which is in turn connected to the T1 line. The interface Serial0 is configured as ip unnumbered Ethernet0 so that it will respond to the same IP address as the Ethernet port. All systems on the network are given host addresses from the 205.217.146.0 network, and a netmask of 255.255.255.0. They also all have their default gateway address set to 205.217.146.1.