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.
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.
Things You'll Need
Router or routable switch
Ethernet or console cable
IP routing is a highly adaptable and complicated process. While this guide provides a general overview of how two subnets are theoretically connected, the actual commands issued and the connections made depend greatly on the network configuration.
Static routes can be used in place of routing protocols if the setup is simple and unlikely to change over time. Static routes are configured by identifying the destination address, the interface through which the data need to be forwarded to reach that network, and any wildcards (wildcard are important when you want to specify a whole range of addresses in a subnet).
Be very careful when starting up routing protocol on complex networks currently in use. Running to many protocols or propagating too much route information can cause outages and downtime throughout the network. If you are not in charge of your network’s configuration, consult with the network administrator or IT consultants.