Any data, information or even hardware which are shareable among different computers are known as resources in computer network.
Resource sharing is the major, reason for interconnecting different computing systems. Such a interconnected system of computers are known as network.
Each computer in this interconnected group of computers is called a host computer.
So a network can be termed as a group of interconnected hosts.
Any work which is running on a system is called a process.
In computer networking major focus is for processes which are participating in the resources sharing between host computers.
example : A process sending URL GET get request from a Chrome browser.
A web server is a machine which delivers resources related to content of website, as per the request by any other machine(clients) in the network.
Both server and client machine are hosts of a same network or different networks.
A URL like http://www.wikipedia.org/wiki/Main_Page has three parts.
http:// - File transfer protocol over world wide web(WWW).
www.wikipedia.org - domain name, an unique human understandable address of the server.
Every domain name has its own machine readable unique numerical address called IP (Internet protocol).
Domain Name Server (DNS) is service that has to be provided by every Internet Service Provider (ISP) to convert domain name into an IP address
/wiki/Main_Page - path: refers to the file location on server.
Let us see what happens when you enter the URL http://www.wikipedia.org/wiki/Main_Page
Your system will contact the Domain Name Server (DNS) of your internet service provider (ISP) to get back the IP address of the server.
Using IP address your request will reach the target server system, and find the required resource.
Contacting a DNS takes some time, which causes for some delay in getting information. The delay is called DNS overhead.
In order to avoid such delay, your system will contact DNS for IP address conversion, only at its first time access of an URL and stores the IP of domain name in your system itself. So unless the IP has not expired there will not be any DNS over head (no time will not be spend for domain name to IP conversion) for next time.
An IP address is a combination of net Network ID and and Host ID.
A n bit binary number can make 2n numbers,
of which if we choose first block of k bits to represent network ID, it will be of 2k networks.
And remaining n-k bit for hosts gives a 2n-k hosts in each network.
Upon reaching at network with NID and then to host( or server in case of web service ) with HID of IP will make connection only till the interface of server machine.
In order to access required resource the system need to know which process is providing the requested resources, for that port number is used.
Port numbers are globally known, and will be different for different protocols.
For Hypertext Transfer Protocol (http) it is 80
For Hypertext Transfer Protocol Secure (https) it is 443
For Simple Mail Transfer Protocol (smtp) it is 25