Networking Fundamentals - NOTES

• A frame is data encapsulated with a destination and source MAC addresses. 
• MAC Address: 48 bits
  eg: 0000.0c47.93c1
  
  

BRIDGING: 

• A device that passes packets between multiple network segments using the same communications media. 
• If a packet is destined for a user within the sender's own network segment, the bridge keeps the packet local. 
• If the packet is bound for another segment, the bridge passes the packet onto the network backbone. 
• In general, a bridge will filter, forward, or flood an incoming frame based on the MAC address of that frame.

It is important to note when bridging a frame that as the frame moves through the internetwork, its destination and source MAC addresses always stay the same. They do not take on the MAC address of the interfaces on the bridge.

Bridges forward broadcast packets. 


ROUTERS AND ROUTING TABLES:

• The device that makes internetworking possible is the router.

• The job of the router is to keep track of which path to use when transferring data from one network to another.

• To determine which routes to use and which routes are the most optimal, routers use a set of rules called routing protocols and store the results in routing tables.

• A packet is data encapsulated with a destination and source network address.

• If the router does not have the destination network in its routing table, it does one of two things: The router either forwards the packet to a predetermined default router, or it drops the packet and informs the sending device that the network is not reachable.

The fundamental concept of routing: As the data moves through the network, the destination and source network addresses stay the same, while the data-link address changes with each different network.

• Routers use routing tables to store information about destinations in the internetwork.

• Some routing protocols maintain an entry for each possible path to a destination. Other protocols maintain only the most desirable path to each destination. 

• Information that a routing table contains includes the destination address, interface, and the desirability of a path. 

• An IP routing table consists of destination address/next-hop pairs. The next hop is the IP address of the router that the outbound packet is handed to. 

• Routing table entries can be interpreted as meaning: To reach network A, send the packet out Node A via interface 0.

• Routers communicate with each other using Routing Updates.

NETWORK ADDRESSING: 

• Unlike link-layer addresses, which usually exist within a flat address space, network-layer addresses are hierarchical.

• End systems require one network-layer address per physical network connection for each network-layer protocol they support.

• A network-layer address will always have a network part and a host part.