Subnetworks, or subnets, are created by taking a single private address range and dividing it into multiple separate networks using a subnet mask. Such division is often used in large companies to help network administrators divide access between different sensitive, network resources. Computers located on different subnets may need to communicate directly with one another. Accomplishing this requires that the two machines be connected to a router, which can forward information based on routable IP addresses.
Connect the computers to the network. Ensure that each connection eventually reaches a router or a routable switch.
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Connect the routers to each other. This step is only necessary if the two separate subnets are connected to two physically separate routers. If the two routers do not have an available, routable interace, they must be connected to a third, interim "core" router, designed to handle routing between the other routers and anything outside of those networks.
Enable a routing protocol in each subnet's router. Options include Routing Information Protocol (RIP), Open Shortest Path First (OSPF) or, on Cisco-based switches, Interior Gateway Routing Protocol (IGRP).
Allow time for the routing tables to update. Routing protocols advertise to neighboring routers the networks to which they are they are directly connected. In this way, each routers gets an images of networks to which they are indirectly connected (i.e. they are connected to a router which is connected to a destination network). When all directly attached routers have up-to-date information about neighboring routers and their attached networks, this is referred to as "convergence." The more complex the network, the longer it takes for convergence to occur.
Log into one of the computers on a subnet and issue a trace route command to the computer on the other subnet. This will show you that communication is functioning properly and that the information is taking the appropriate path (each routed interface, or "hop," will be listed as part of the route the packet took). To issue a traceroute in Windows, open the command prompt and type "tracert [IP address]", where [IP address] is the address of the computer on the other subnet.