When you write a letter to someone, you usually put a complete address
on the envelope, specifying the country, state, zip code, etc. After
you put it into the letter box, the postal service will deliver it to
its destination: it will be sent to the country indicated, whose national
service will dispatch it to the proper state and region, etc. The
advantage of this hierarchical scheme is rather obvious: Wherever you
post the letter, the local postmaster will know roughly the direction to
forward the letter to, but doesn't have to care which way the letter
will travel by within the destination country.
IP-networks are structured in a similar way. The whole Internet
consists of a number of proper networks, called autonomous systems.
Each such system performs any routing between its member hosts
internally, so that the task of delivering a datagram is reduced to finding
a path to the destination host's network. This means, as soon as the datagram
is handed to any host that is on that particular network, further
processing is done exclusively by the network itself.