Imagine having to travel in Paris, France, from the Vendome to the Eiffel Tower. This path would require us to use a map and review the streets and turns we would take. In networking, packets don’t get a map and aren’t smart enough to review the path ahead of time.  They go to where they are directed, the next Router in the path.  For us, it would be similar to driving to the gas station asking for directions to the next gas station in the general direction of the Eiffel Tower.  We would repeat this gas station based driving until we reach our destination.  (Note:  the car driver is not male… 😉 )  The gas station attendant would look up the map for us and point us in the right direction.  Packets use a Routed Protocol that has logical network and host portions. Routers use the network portion to create a Routing Table to direct packets.  The Routing Table is updated by Routing Protocols.  The Default Gateway (DGW) is the first router in the path.  The packet has the DGW defined by DHCP or administratively.

Maps versus Routing

Now where are we going in our car is typically a street address and exact location.  In IP routing, it’s a subnet, a general location that will be defined to exact the by the local default gateway (the local router).  Finally how do the gas stations know about other gas stations to give us directions to them, that would be a map that needs to be updated everytime a new location is defined.  In IP routing, thats a Routing Protocol used to update all the routers.

• Routed Protocol

A packet based solution that uses hierarchical logical addressing where a network portion of the address provides a general location, like a city. The host portion defines the exact location, like a street address.
Examples Appletalk, IPX, IPv4 and IPv6.

• Router

Network device that resides in Layer 3 and handles the forwarding of packets from one subnet to another.  Routers are the default gateways for endpoints and stops the propagation of broadcasts.

• Routing Protocol

Routing protocol is used by routers to update each other about locations/ subnets that are available and how to proceed to them.
Examples RIP, RIPv2, EIGRP, and OSPF.

• Default Gateway

A default gateway is defined on end points to send traffic that is not for the local network.  This traffic is identified by just looking at the network portion of the address to determine if it’s local or not.

Routing is typically defined into three types

1. Static Routing
2. Default Routing
3. Dynamic Routing

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