One of the basic functions provided by the IP protocol is the ability to form
connections between different physical networks. A system that performs this
function is called an IP router. This type of device attaches to two or more
physical networks and forwards datagrams between the networks.

When sending data to a remote destination, a host passes datagrams to a local
router. The router forwards the datagrams toward the final destination. They
travel from one router to another until they reach a router connected to the
destination’s LAN segment. Each router along the end-to-end path selects the
next hop device used to reach the destination. The next hop represents the next
device along the path to reach the destination. It is located on a physical network
connected to this intermediate system. Because this physical network differs
from the one on which the system originally received the datagram, the
intermediate host has forwarded (that is, routed) the IP datagram from one
physical network to another.

The IP routing table in each device is used to forward packets between network
segments. The basic table contains information about a router’s locally
connected networks. The configuration of the device can be extended to contain
information detailing remote networks. This information provides a more
complete view of the overall environment.

A robust routing protocol provides the ability to dynamically build and manage
the information in the IP routing table. As network topology changes occur, the
routing tables are updated with minimal or no manual intervention.

Note: Early IP routing documentation often referred to an IP router as an IP
gateway.

Autonomous systems

The definition of an autonomous system (AS) is integral to understanding the
function and scope of a routing protocol. An AS is defined as a logical portion of
a larger IP network. An AS normally consists of an internetwork within an
organization. It is administered by a single management authority.an AS can connect to other autonomous systems managed by the same organization. Alternatively, it can connect to other public or private networks.

Some routing protocols are used to determine routing paths within an AS. Others
are used to interconnect a set of autonomous systems:

Interior Gateway Protocols (IGPs):
Interior Gateway Protocols allow routers to exchange information within an AS. Examples of these protocols are Open Short Path First (OSPF) and Routing Information Protocol (RIP).

Exterior Gateway Protocols (EGPs):

Exterior Gateway Protocols allow the exchange of summary information between autonomous systems. An example of this type of routing protocol is Border Gateway Protocol (BGP).

the Interior Gateway Protocols used to maintain routing information within each AS. The Exterior Gateway Protocols maintains the routing information between autonomous systems.

Within an AS, multiple interior routing processes can be used. When this occurs,
the AS must appear to other autonomous systems as having a single coherent
interior routing plan. The AS must present a consistent view of the internal
destinations.

Types of IP routing and IP routing algorithms

Routing algorithms build and maintain the IP routing table on a device. There are
two primary methods used to build the routing table:

Static routing:

Static routing uses preprogrammed definitions representing paths through the network.

Dynamic routing:

Dynamic routing algorithms allow routers to automatically discover and maintain awareness of the paths through the network. This automatic discovery can use a number of currently available dynamic routing protocols. The difference between these protocols is the way they discover and calculate new routes to destination networks. They can be classified into four broad categories:

– Distance vector protocols
– Link state protocols
– Path vector protocols
– Hybrid protocols

There are several reasons for the multiplicity of protocols:

Routing within a network and routing between networks typically have
different requirements for security, stability, and scalability. Different routing
protocols have been developed to address these requirements.

New protocols have been developed to address the observed deficiencies in
established protocols.

Different-sized networks can use different routing algorithms. Small to
medium-sized networks often use routing protocols that reflect the simplicity
of the environment.However, these protocols do not scale to support large, interconnected networks.More complex routing algorithms are required to support these environments.

If you want more information look into my other blogs in my profile. Also one very good website for networking tutorials specially for CCNA is
http://ccna2ccna.googlepages.com