Some Basic CCNA Exam Preparation Topics Part I

OK let us list some basic network theory that you will be required to know by heart for your CCNA preparation.

We have three types of data transmission:
- Unicast: This is the most common transmission type. Data flows from a single source host to a single destination host.
- Multicast: Data flows from a source host to a Group of destination hosts that have “registered” to listen to the transmission. This is like a conference call where users can join a conference in order to listen what the talker has to say. Multicast works with special IP addresses in the range 224.0.X.X.
- Broadcast: Data flows from a single source host to ALL other hosts on the same network. For example if the local LAN is subnet 192.168.1.0/24, then broadcast packets have a destination address 192.168.1.255. By default a router will block broadcast packets and not let them travel beyond the local network.

A Broadcast Domain is a common Layer 2 segment where all devices receive broadcast frames. For example several hosts connected on the same Cisco switch and having the same layer 2 vlan, belong to the same broadcast domain. A router acts as the boundary of a broadcast domain.
A Collision on an Ethernet link occurs when two devices send data simultaneously on the wire, thus data collision occurs. A Collision Domain is the area in an Ethernet network over which collisions will be detected. For example a switch with 24 ethernet ports has 24 collision domains. Collisions are propagated by hubs and repeaters but are not propagated by routers or switches.
An Ethernet Hub is working at Layer 1 of the OSI model and all devices connected on a Hub belong to the same Collision Domain (and same Broadcast Domain as well).
An Ethernet Switch works at Layer 2 of the OSI model. All devices connected on a switch belong to the same Broadcast Domain but each switch segment (i.e port of the switch) has its own collision domain.
A Router works at Layer 3 of the OSI model. A router blocks network broadcasts. Some of the tasks of a Layer 3 router are broadcast and multicast control, traffic management, optimal path determination, logical addressing and connecting to WAN services.