Think Different: CCNA - Network Basics

CCNA

Cisco Certified Network Associate

Introduction to Networking and Network Basics :

It is important to understand what a network is and the importance of networks themselves. Every host has a Network Interface Card (NIC) that is used to connect it to a network.

A hub is a network device that repeats information received from a host to all other connects hosts. The hub will relay any information received from Host A to Host B and Host C. This means that all the three hosts can communicate with each other. Communication between hosts can be classified into 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 few hosts only.

When a hub is used to network hosts, there are two problems that arise:

A hub repeats information received from one host to all the other hosts. To understand this, consider Host A 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. Even though the message was a unicast intended only for Host B, Host C also receives it. It is up to Host C to read the message and discard it after seeing that the message was not intended for it.

A hub creates a shared network 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 called a single collision domain. Imagine the impact of having a single collision domain where 50 or 100 hosts are connected to hubs that are interconnected and they are all trying to send data. That is just a recipe for many collisions and an inefficient network.

The problems associated with hubs can cause severe degradation of a network. To overcome these, switches are used instead of hubs. Like hubs, switches are used to connect hosts in a network but switches break up collision domain by providing a single collision domain for every port. This means that every host (one host connects to one port on the switch) gets its own collision domain thereby eliminating the collisions in the network. With switches, each host can transmit 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.

Now that you know how a switch works and improves a network, 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 network. To remedy this situation, networks are broken into smaller sizes and these separate 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.