Hypertext Transfer Protocol, better known as HTTP, is the protocol that web clients and web servers use to communicate. It is the "language" that clients and servers speak to one another in order to exchange information.
HTTP defines how messages are formatted and transmitted, which determines how web clients (i.e. web browsers) and servers respond to various commands.
HTTP is known as a request-response protocol. This means that a client makes an HTTP request to a web server asking for a web resource (i.e. an HTML page), and the server will return the resource in its HTTP response message.
The Domain Name System (DNS) is a system for translating website names into numbers (IP addresses). Every website and internet-connected device has an associated IP address. PentesterLab's IP address, for example, is 54.172.242.130.
Evidently, long numbers like these is hard to remember, and DNS provides a solution to this. Instead, a user is able to access PentesterLab by visiting pentesterlab.com — DNS provides the functionality that syncs human-readable website addresses with their associated IP addresses.
On entering a website address into the URL bar in a browser, this triggers the DNS server to search through its database and look for a matching IP address to the one entered. Once found, the server will resolve that domain name to the IP address.
WHOIS is a protocol used for querying databases for information regarding a website domain name, including who owns the domain and the registrar that the domain was registered with.
There are multiple ways to use WHOIS. One method is to visit the WHOIS website, another use the whois command in command line, like so:
$ whois pentesterlab.com
A network socket is an internet endpoint which allows a user to connect to a server (i.e. a remote computer). Both web clients and web servers each have a socket, IP address, and port number. The server will have a consistent port number, whilst the client is assigned a randomly generated number (known as an ephemeral port), typically between 1024 and 65535.
Generally, a "socket" refers to a socket in an Internet Protocol (IP) network. HTTP requests and responses are built on sockets.
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