The OSI Model is a conceptual model or framework describing how two computing devices establish and maintain communications to transfer information between them. It is the international standard for inter-computer communication.
A subnet is a physical segment of a TCP/IP network that uses IP addresses derived from a single Network ID. Large networks are typically divided into several subnets for ease of management and sometimes because of physical limitations.
Website design deals with the structure and content of your site. The most important step in planning your site is to organize your information and this requires that you think carefully about what you want to say and how you want to say it.
web page design deals with the content and layout of the individual pages within your site. It is concerned with what goes on the pages, how it is placed on the page, and where it is placed on the page.