The Network Layer

In essence, the Network Layer is responsible for end-to-end (source to destination) packet delivery including any routing through intermediate hosts, whereas the link layer is responsible for node-to-node (hop-to-hop) frame delivery on the same link.

The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service, and error control functions.

The Network Layer deals with transmitting information all the way from its source to its destination - transmitting from anywhere, to anywhere. Here are some things that the Network Layer needs to address:

Is the network connection-oriented or connectionless?
For example, snail mail is connectionless, in that a letter can travel from a sender to a recipient without the recipient having to do anything. On the other hand, the telephone system is connection-oriented, because the other party is required to pick up the phone before communication can be established. The OSI Network Layer protocol can be either connection-oriented, or connectionless. The IP Internet Layer (equivalent to OSI's Network Layer) supports only the connectionless Internet Protocol (IP); however, connection-oriented protocols, such as TCP, exist higher in the stack by enforcing reliability constraints through timeouts and resending packets.

What are the Global Addresses?
Everybody in the network needs to have a unique address which determines who he is. This address will normally be hierarchical, so you can be "Fred Murphy" to Dubliners, or "Fred Murphy, Dublin" to people in Ireland, or "Fred Murphy, Dublin, Ireland" to people anywhere in the world. On the internet, these addresses are known as IP Addresses.

How do you forward a message?
This is of particular interest to mobile applications, where a user may rapidly move from place to place, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) doesn't really allow for this, though it has been hacked somewhat since its inception. Fortunately, the forthcoming IPv6 has a much better designed solution, which should make this type of application much smoother.