ELSA Cable manual IP routing and hierarchical IP addressing

Technical basics 55

Routing

Routing-table

Hierarchical IP addresses

There are two considerations when using these IP addresses:

KThe IP addresses used in a private network should not leave this network, i.e. an Internet connection is only possible when using IP masquerading, for example.

KThe packets for these IP addresses will not be routed in the Internet, i.e. backbone routers will simply reject such IP packets. Depending on the provider, serious consequences may result if such IP packets are released on the Internet.

IP routing and hierarchical IP addressing

Every IP packet contains the IP addresses of the source and of the recipient. A router receives IP packets at its interface, interprets the destination address and passes the packets on to those interfaces that are nearest to the recipient. Finding the appropriate path is called routing.

Every router manages a table (routing table). This table indicates the quickest interface connection to the host for every host in the network. You can imagine that, as they grow, these tables may exceed the capacity of the router—the Internet, as a worldwide linking up of publicly accessible IP computers contains several millions of hosts.

For this reason, hierarchical IP addresses were introduced. This means dividing the IP network into subnets in which IP addresses are appointed from a coherent numerical range. It is possible to establish several hierarchy levels, so that different subnets can be merged. The principle is similar to the hierarchical address used by paper mail, consisting of a country, a city, a street and a number.

The consequences of this hierarchical IP addressing:

KSince all hosts within one network have the same network address, the host address is sufficient for the hosts within this network to communicate with one another.

KA router has to know both the addresses of the hosts that are directly connected to it, and the addresses of all networks and subnets that are reachable via adjacent routers.

KIt is not necessary for a router to know all other possible IP addresses.

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