What is the main reason the OSI Model was created? 

The speed of the transmission data is generally determined by the cables or connectors we use for data transmission. In networking, layer-1 the physical is which deals with the network cables like pins, usb, connectors, etc... we use for data transmission.

The network layer concerns with the packets. It brings the packets from the source to the destination.

Application layer protocol defines types of messages exchanged, message format, syntax and semantics and rules for when and how processes send and respond to messages.

Ethernet uses Bus topology.

A network is nothing but 2 or more computers are grouped together to share information and resources and the that arrangement is called a topology.

Cluster network topology is not a common type.

The logical arrangement of the nodes of different networks to communicate is called as topology.

There are two IP address assigned to each device on a Wi-Fi network.

An Internal (Private) and an External (Public) IP address.

Internal IP address is used for communication between Your computer and a smartphone or whatever devices that are connected to that Wifi network. This is unique for each. If they are same the system cannot differentiate between each device.

While, the External IP address is that visible to the the internet - the websites, the internet connected Apps so on. By default, Routers with NAT (all routers today have it) assign same Public IP address to all devices under its network.