|  | 11.1 | Layer 3 Devices
| 11.1.3 | Unique network numbers

Routers connect two or more networks, each of which must have a unique network number in order for routing to be successful. The unique network number is incorporated into the IP address that is assigned to each device attached to that network.

Example:
A network has a unique network number - A. It has four devices attached to it. The IP addresses of the devices are A2, A3, A4, and A5. Since the interface where the router connects to a network is considered to be part of that network, the interface where the router connects to network A has an IP address of A1.

Example:
Another network, with a unique network number - B - has four devices attached to it. This network is also attached to the same router, but at a different interface. The IP addresses of the devices on this second network are B2, B3, B4, and B5. The IP address of the router's second interface is B1.

Example:
You want to send data from one network to another. The source network is A; the destination network is B; and a router is connected to networks A, B, C, and D. When data (frames), coming from network A, reaches the router, the router performs the following functions:

1. It strips off the data link header, carried by the frame. (The data link header contains the MAC addresses of the source and destination.)
2. It examines the network layer address to determine the destination network.
3. It consults its routing tables to determine which of its interfaces it will use to send the data, in order for it to reach its destination network.

In the example, the router determines that it should send the data from network A to network B, from its interface, with address B1. Before actually sending the data out interface B1, the router would encapsulate the data in the appropriate data link frame.