What is inter-network routing?

In this article we shall discuss about inter-network routing. Before moving to that there are certain terms with which you should be familiar:

Ø  End systems: The ISO (the international standards of organization) defines the end systems as the network elements that do not have the ability of forwarding the packets across the networks. Sometimes the term host is used to refer to the end systems.

Ø  Intermediate systems: These are the network elements that have the ability of forwarding the packets across the network. Most common examples are routers, switches, bridges and so on.

Ø  Network: It can be defined as a part of the inter-network infrastructure encompassing various elements including hubs, repeaters, bridges and so on. The networks are bounded by the intermediate systems.

Ø Router: This is one of the intermediate systems that is used for connecting various networks with each other. It might support one protocol (router) or many protocols (multi-protocol router). Its hardware part is optimized especially for performing routing. The software part is responsible for carrying out the routing and takes care of the routing tables.

Apart from these devices, there are 3 types of addresses involved in inter-network routing:

Ø  The inter-network address: The host address and the network address are combined together to form this address. This is used for unique identification of a host over the inter-network.

Ø The host address or host ID: This ID might be assigned by the administrator or might be simply the physical address of the host. It is used for the unique identification of the host on its network.

Ø  The network address or network ID: This is address of a network for identifying it in an inter-network.

All the data packets consist of a network layer header. This network layer header consists of the following when the packet is transmitted from one host to another:

ØThe address of the source inter-network: This address combines the address of the source host and the source network.

ØThe address of the destination inter-network: This address combines the address of the destination host and the destination network.

ØThe hop count: This usually begins at zero and is numerically incremented when the packet crosses a router. Or in the opposite case it might be assigned some maximum value which might be decremented on reaching a router. The purpose of using the hop count is to make sure that the packet does not keeps on circulating endlessly in the network.

- For inter-network routing, two things have to be known.

- Firstly, how do you reach other routers which lie in the same network and secondly, how do you reach other routers which lie in other networks? 

- The answer to the first question is easy as it is the common routing problem among two hosts residing over the same network. 

- This routing is handled by the interior gateway protocol and it is different for different networks since only local routing info is required. 

- In this case, the commonly used protocol is the open shortest path first or OSPF protocol. 

- The routing between two different networks is performed using the exterior gateway protocol. 

- This is actually the problem of inter-network routing. 

- Here, the commonly used protocol is the BGP or the border gateway protocol. 

- The graph for inter-network routing is quite different from the one that is used in the network routing. 

- This is so because the routers which lie in the same network can be thought of as being directly connected to one another for routing across inter-network. - All the networks in an inter-network function as though they are one large unit.