Topology defines the structure of the network.
There are two parts to the topology definition: the physical topology, which is the actual layout of the
wire (media), and the logical topology, which defines how the media is accessed by the hosts. The physical topologies that are commonly used are the Bus, Ring, Star, Extended Star, Hierarchical, and Mesh. These are shown in the graphic. 
- A bus topology uses a single backbone segment (length of cable)
that all the hosts connect to directly.
- A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.
- A star topology connects all cables to a central point of concentration. This point is usually a hub or switch, which will
be described later in the chapter.
- An extended star topology uses the star topology to be created. It links
individual stars together by linking the hubs/switches. This, as you will
learn later in the chapter, will extend the length and size of the network.
- A hierarchical topology is created similar to an extended star but instead of linking the hubs/switches together, the system is linked to a computer that controls the traffic on the topology.
- A mesh topology is used when there can be absolutely no break in
communications, for example the control systems of a nuclear power
plant. So as you can see in the graphic, each host has its own connections to all other hosts. This
also reflects the design of the
Internet, which has multiple paths to any one location.
The logical topology of a network is how the hosts communicate across the medium. The two
most common types of logical topologies are Broadcast and Token-passing.
Broadcast topology simply means that each host sends its data to all other hosts on the network medium. There is no order the stations follow to use the network, it is first come, first serve.
This is the way that Ethernet works and you will learn much more about this later in the
semester.
The second type is token-passing. Token-passing controls network access by passing an electronic token sequentially to each host.
When a host
receives the token, that means that that host can send data on the network. If the host has no data to send, it passes the token to the next host and the process repeats itself.
The diagram in the graphic shows many topologies. 
It shows a LAN of moderate complexity that is typical of a school or a small business. It has many symbols, and it depicts many networking concepts that will take time to learn.
This LAN is typical of a small campus, and represents most of the devices that you will study
for your CCNA.
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