Determine the path between two hosts across a network

 Once you create an internetwork by connecting your WANs and LANs to a router, you'll need to configure logical network addresses, such as IP addresses, to all hosts on the internetwork so that they can communicate across that internetwork.

The term routing is used for taking a packet from one device and sending it through the network to another device on a different network. Routers don't really care about hosts—they only care about networks and the best path to each network. The logical network address of the destination host is used to get packets to a network through a routed network, and then the hardware address of the host is used to deliver the packet from a router to the correct destination host.

Eventually the packet reaches a router that is part of a network that matches the destination IP address of the packet. In this example, router R2 receives the packet from R1. R2 forwards the packet out its Ethernet interface, which belongs to the same network as the destination device, PC2.

Routers Operate at Layers 1, 2, and 3

A router makes its primary forwarding decision at Layer 3, but as we saw earlier, it participates in Layer 1 and Layer 2 processes as well. After a router has examined the destination IP address of a packet and consulted its routing table to make its forwarding decision, it can forward that packet out the appropriate interface toward its destination. The router encapsulates the Layer 3 IP packet into the data portion of a Layer 2 data link frame appropriate for the exit interface. The type of frame can be an Ethernet, HDLC, or some other Layer 2 encapsulation – whatever encapsulation is used on that particular interface. The Layer 2 frame is encoded into the Layer 1 physical signals that are used to represent bits over the physical link.

To understand this process better, refer to the figure. Notice that PC1 operates at all seven layers, encapsulating the data and sending the frame out as a stream of encoded bits to R1, its default gateway.

R1 receives the stream of encoded bits on its interface. The bits are decoded and passed up to Layer 2, where R1 de-capsulate the frame. The router examines the destination address of the data link frame to determine if it matches the receiving interface, including a broadcast or multicast address. If there is a match with the data portion of the frame, the IP packet is passed up to Layer 3, where R1 makes its routing decision. R1 then re-encapsulates the packet into a new Layer 2 data link frame and forwards it out the outbound interface as a stream of encoded bits.

R2 receives the stream of bits, and the process repeats itself. R2 de-capsulate the frame and passes the data portion of the frame, the IP packet, to Layer 3 where R2 makes its routing decision. R2 then re-encapsulates the packet into a new Layer 2 data link frame and forwards it out the outbound interface as a stream of encoded bits.

This process is repeated once again by router R3, which forwards the IP packet, encapsulated inside a data link frame and encoded as bits, to PC2.

Each router in the path from source to destination performs this same process of de-capsulation, searching the routing table, and then re-encapsulation. This process is important to your understanding of how routers participate in networks. Therefore, we will revisit this discussion in more depth in a later section.

Routing Table Principles



At times in this course we will refer to three principles regarding routing tables that will help you understand, configure, and troubleshoot routing issues. These principles are from Alex Zinin’s book, Cisco IP Routing.

1. Every router makes its decision alone, based on the information it has in its own routing table.

2. The fact that one router has certain information in its routing table does not mean that other routers have the same information.

3. Routing information about a path from one network to another does not provide routing information about the reverse, or return, path.

What is the effect of these principles? Let’s look at the example in the figure.

1. After making its routing decision, router R1 forwards the packet destined for PC2 to router R2. R1 only knows about the information in its own routing table, which indicates that router R2 is the next-hop router. R1 does not know whether or not R2 actually has a route to the destination network.

2. It is the responsibility of the network administrator to make sure that all routers within their control have complete and accurate routing information so that packets can be forwarded between any two networks. This can be done using static routes, a dynamic routing protocol, or a combination of both.

3. Router R2 was able to forward the packet toward PC2’s destination network. However, the packet from PC2 to PC1 was dropped by R2. Although R2 has information in its routing table about the destination network of PC2, we do not know if it has the information for the return path back to PC1’s network.

Difference between LAN and WAN

     Key Difference: LAN is a computer network that connects computers in small areas. WAN is a network that covers a broad area using private or public network transports.

The terms LAN and WAN are often confusing for people that aren’t that tech savvy. These are both connections that allow users to connect their computer to a network, including the internet. LAN is short for Local Area Network, while WAN is short for Wide Area Network. These two differ from each other in distinct ways.

 LAN is a computer network that connects computers in small areas such as home, office, school, corporation, etc. using a network media. It is useful for sharing resources such as printers, files, games, etc. A LAN network includes a couple of computer systems connected to each other, with one system connected to a router, modem or an outlet for internet access. The LAN network is built using inexpensive technologies such as Ethernet cables, network adapters and hubs. However, other wireless technologies are also available to connect the computer through a wireless access. In order to configure a LAN network, a person may also require specialized operating system software. The most popular software includes the Microsoft Windows’ Internet Connection Sharing (ICS), which allows users to create LAN.

 The first successful LAN network was created by Cambridge University in 1974 known as the Cambridge Ring; however it was not commercialized until 1976 by Datapoint Corporation. Datapoint’s ARCNET was installed at Chase Manhattan Bank in New York in 1977. The main purpose of creating a LAN was to share storage and other technologies such as printers, scanners, etc. The smallest LAN can include two computers, while the largest can, in theory, support 16 million devices according to About.com. Wikipedia states that “the larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loops, their ability to manage differing traffic types via quality of service (QoS), and to segregate traffic with VLANs.” The larger LANs also employ other devices such as switches, firewalls, routers, load balancers, and sensors.

WAN is a network that covers a broad area using private or public network transports. The best example of WAN would be the Internet, which can help connect anyone from any area of the world. Many businesses and government use WAN in order to conduct business from anywhere in the world. WANs are also responsible largely for businesses that happen across the world (i.e. a company in UK does business with a company in China). The basic definition of WAN includes a network that can span regions, countries, or even the world. However, in practicality, WAN can be viewed as a network that is used to transmit data over long distances between different LANs, WANs and other networking architectures.

WANs allow the computer users to connect and communicate with each other regardless of location. WAN uses technologies such as SONET, Frame Relay, and ATM. WANS allow different LANs to connect to other LANs through technology such as routers, hubs and modems. There are four main options for connecting WANs: Leased line, Circuit switching, Packet switching and Call relay. Leased lines are point-to-point connection between two systems. Circuit switching is a dedicated circuit path between two points. Packet switching includes devices transporting packets via a shared single point-to-point or point-to-multipoint link across a carrier internetwork. Call relay is similar packet switching but uses fixed length cells instead of variable length packets.
detailed description is available below:

	LAN	WAN
Definition	LAN is a computer network that connects computers in small areas.	WAN is a network that covers a broad area using private or public network transports.
Data transfer rates	LAN offers high data transfer rates.	WAN has lower data transfer rates due to congestion
Speed	80-90 mbps	10-20 mbps
Technology	LANs use technologies such as Ethernet and Token Ring to connect to other networks.	WAN uses technologies such as MPLS, ATM, Frame Relay and X.25 for data connection over greater distances.
Bandwidth	High bandwidth is available for transmission.	Low bandwidth available for transmission.
Connection	One LAN can be connected to other LANs over any distance via telephone lines and radio waves.	Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites.
Components	Layer 2 devices like switches, bridges. Layer 1 devices like hubs, repeaters.	Layers 3 devices Routers, Multi-layer Switches and Technology specific devices like ATM or Frame-relay Switches etc.
Problems	LANs tend to have fewer problems associated with them.	WANs have more problems  due to the large amount of system and data that is present.
Ownership	LAN networks can be owned up private companies or people that set it up at homes.	WAN are not owned up any one organization but exist under collective or distributed ownership.
Data Transmission Error	Experiences fewer data transmission errors.	Experiences more data transmission errors.
Cost	Set-up costs are low as the devices required to set up the networks are cheap.	Set-up costs are high, especially in remote locations where set-up is not done. However, WANs using public networks are cheap.
Spread	The network is spread to a very small location.	The network can be spread world-wide.
Maintenance costs	Maintenance costs are low as the area coverage is small.	Maintenance costs are high as the area coverage is world-wide.
Congestion	Less congestion	More congestion

 LANs are become more and more common in many places such as offices, corporations, homes, etc. A main reason for their growing popularity is that they are cheaper to instill and offer higher transfer speeds. LANs offer speeds up to 80 or 90 mbps due to the proximity of the computer systems to each other and lack of congestion in the network. In comparison, WANs can provide a speed of 10 to 20 mbps. Also LANs offer better security compared to WANs, which are more easily accessible with the people that know how to hack systems. WANs and LANs can be secured using firewalls, anti-virus and spyware softwares.