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.

Related posts:

1. CCNA Certification Exam Preparation Topics Part 2 – Router Passwords
2. CCNA 640-802 Tutorial: seven-layer OSI Model
3. Understanding the Transport Layer and Port Numbers CCNA Tutorial
4. CCNA Preparation – What are the Best Study Methods and Resources
5. How Hosts Use Subnet Masks
6. Forwarding a DHCP Request using Cisco ip helper-address command
7. Cisco IOS Router Operation-CCNA Exam Prep