When you need to move data between two networks, an internetworking device called a router is responsible for the movement of this data. Routing data on an internetwork requires that a couple different events take place: an appropriate path for the packets must be determined, and then the packets must be moved toward their final destination. That is, routers perform two basic activities: path determination and packet switching. Both path determination and routing of packets (or switching as it is also referred to; packets are switched from an incoming interface to an outgoing interface on the router) take place at layer 3 (network layer) of the OSI Model. Another important layer 3 event is the resolution of logical addresses (such as IP addresses when TCP/IP is the routed protocol) to actual hardware addresses.
Path Determination
Routers enable you to divide a large network into logical subnets; doing so keeps network traffic local on each subnet, enabling you to take better advantage of the available bandwidth. It is then the job of the router to move data packets between different subnets when required. Routers can also serve as a connection device between your networks. Routers can also serve as the connective device to other networks to which your network is attached.
Packet Switching
After the router has the packets, packet switching comes into play. This means that the router will move the packets from the router interface that they came in on and switch them over to the router interface connected to the subnet they must go out on. However, in some cases, the packets might have to pass through more than one router to reach the final destination. In our example, there is only one router. Router 1 knows that the logical address 10.16.0.1 is on Subnet 10.16. So, the router will switch packets from Router Interface 1 to Router Interface 2. Again, the router uses broadcast messages to resolve logical address 10.16.0.1 to the actual hardware address of FF-FF-FF-FF-FF-F5. The router addresses the
packets correctly and then forwards them to Subnet 10.16. When Node 1 on Subnet 10.16 sees the packets for itself (hardware address FF-FF-FF-FF-FF-F5), it copies the messages into its memory space.
Routing Tables for Router 1
Subnet Logical Destination Router Interface
10.8 1
10.16 2
Routing involves the use of logical addresses and hardware addresses to get packets from the source (i.e., the sender) to the destination. Each routable protocol (e.g., IP and IPX) uses a slightly different method for resolving logical addresses to hardware addresses, but the overall theory is pretty much the same as outlined here.
Path Determination
Routers enable you to divide a large network into logical subnets; doing so keeps network traffic local on each subnet, enabling you to take better advantage of the available bandwidth. It is then the job of the router to move data packets between different subnets when required. Routers can also serve as a connection device between your networks. Routers can also serve as the connective device to other networks to which your network is attached.
Packet Switching
After the router has the packets, packet switching comes into play. This means that the router will move the packets from the router interface that they came in on and switch them over to the router interface connected to the subnet they must go out on. However, in some cases, the packets might have to pass through more than one router to reach the final destination. In our example, there is only one router. Router 1 knows that the logical address 10.16.0.1 is on Subnet 10.16. So, the router will switch packets from Router Interface 1 to Router Interface 2. Again, the router uses broadcast messages to resolve logical address 10.16.0.1 to the actual hardware address of FF-FF-FF-FF-FF-F5. The router addresses the
packets correctly and then forwards them to Subnet 10.16. When Node 1 on Subnet 10.16 sees the packets for itself (hardware address FF-FF-FF-FF-FF-F5), it copies the messages into its memory space.
Routing Tables for Router 1
Subnet Logical Destination Router Interface
10.8 1
10.16 2
Routing involves the use of logical addresses and hardware addresses to get packets from the source (i.e., the sender) to the destination. Each routable protocol (e.g., IP and IPX) uses a slightly different method for resolving logical addresses to hardware addresses, but the overall theory is pretty much the same as outlined here.