What is Network?

Before starting the Cisco Internetworking, the network introduction is important for the student of networking to be aware of “what is network” and “what is network importance themselves”.

So, first of all, what is network?

In simple words, It is a collection of interconnected devices (such as computers, printers, etc.) in such a way that they can communicate with each other. To better understand, let us look at the example, how things worked before networks. For this, suppose a large international company that sells ABC products at a time when networks did not exist.

Let us call this company XYZ Inc. to see in your mind’s eye the amount of information such as sales, inventory, account, etc. required by the management of the company to make everyday decisions. To get this information they will need to call their local offices. Their local offices will need postal mail or faxing from your email for printed reports or even send media (floppies!) through the postal service. This is a long and time taken process. This job also increases chance error since large numbers of reports are manually processed. This is just one part. You also need reflecting on the information required by the local offices. They also need various data from the head office and other offices around the world.

Now think the same company, but in the present time with all their offices interrelated through the networks. They would use a single application around the world that takes advantage of their global networks. The data from all offices would be instantly stored at the central site, the administration team can see data from around the world in any format. This data would also be real-time. This means that they see it as it’s happening. After centralizing the data, any site office can see data of any location.

The cost, time and effort involved in transferring data were much higher without networks. So networks decrease cost, time and effort and thereby increase output. They also help in resource optimization by helping to share resources.

Now you are familiar with how beneficial networks are, it’s time to look at how networks work. The figure below shows the most basic form of a network. This figure shows two hosts directly connected to each other using a networking cable. Today every host has a NIC for connectivity.

One end of the cable connects to the NIC on a host A  and another end to host B. At this stage do not worry about cables and how the hosts communicate across the network. We will discuss this in detail later in the chapter. At this stage, it is important to understand how hosts connect to a network.

In Figure below, the hosts are “networked” and can send information to each other. This network is successful, but not scalable. If you have more than 2 hosts to this “network”, it will not work without a separate NIC card for each connection and that is not scalable or realistic. For more than 2 hosts to be networked, you need a device such as a hub. The figure below shows three hosts connected to a hub.

In the figure above the hub will relay any information received from Host-A to both Host-B and C. This means that all the three hosts can communicate with each other. When network hosts connect using a hub, there are two problems that arise:

1. A hub repeats information received from one host to all the other hosts. To understand this, consider Host-A in the above network sending a unicast message to Host-B. When the hub receives this message; it will relay the message to both Host-B and Host-C.
2. A hub creates a shared networking medium where only a single host can send packets at a time. If another host attempts to send packets at the same time, a collision will occur. Then each device will need to resend their packets and hope not to have a collision again. This shared network medium is a single collision domain. 

The problems related to hubs can slow the process. To overcome these, use switches instead of a hub. Similar to hubs, switches connect hosts to one another but switches break up collision domain by providing a single collision domain for each port. This means that every host gets its own collision domain thereby eliminating the collisions in the network. With switches, each host can send data anytime. Switches simply “switch” the data from one port to another in the switched network. Also, unlike hubs, switches do not flood every packet out all ports. They switch a unicast packet to the port where the destination host resides. They only flood out a broadcast packet. The figure below shows a switched network.

Remember that each host in Figure above is in its own collision domain and if Host-A sends a packet to Host-C, Host-B will not receive it. Communication between hosts connected to switch is three types:

• Unicast – Communication from one host to another host only.
• Broadcast – Communication from one host to all the hosts in the network.
• Multicast– Communication from one host to a few hosts only

The figure below shows a network See if you can figure out how many collision domains exist in the network.

If you answered 6 then you are absolutely correct since each port of the Switches represents a single collision domain. If you answered more than 5 then you need to remember that a hub does not break collision domains.

Now that you know how a switch works, consider the one problem associated with a switched network. Earlier, you learned that hubs flood out all packets, even the unicast ones. A switch does not flood out unicast packets but it does flood out a broadcast packet. All hosts connected to a switched network are said to be in the same broadcast domain. All hosts connected to it will receive any broadcast sent out in this domain.

While broadcasts are useful and essential for network operations, in a large switched network too many broadcasts will slow down the networking process. To remedy this situation, networks are broken into smaller sizes and these separate sub-networks are interconnected using routers. Routers do not allow broadcasts to be transmitted across different networks it interconnects and hence effectively breaks up a broadcast domain.

In the network shown in Figure below, broadcasts from hosts connected to one switch will not reach to hosts connected to another switch. This is because the router will drop the broadcast on its receiving interface.

In addition to breaking up broadcast domains, routers also do the following essential functions in any network:

Packet Switching

At the barest least, routers are like switches because they essentially switch packets between networks.

Path Selection 

Routers can talk to each other to learn about all the networks connected to various routers and then select the best path to reach a network.

Packet Filtering

Routers can drop or send packets based on certain criteria like their source and destination. This is also discussed in detail later in the book.