Fourth: IP addresses, domain names and DNS

The IP address

• An IP address is a unique identifying number given to every device connected to the Internet; it is the device's "address" on the network.
• IPv4 is divided into four blocks of 8 bits each, separated by a dot and written in decimal from 0 to 255 — for example 192.168.0.1.
• Public addresses are reachable on the Internet and are assigned by the authority; private addresses are used inside a local network and are not routable on the Internet.
• With the rapid spread of the Internet, new IPv4 addresses are almost exhausted — the solution is IPv6, whose 128-bit addresses give an enormous space, together with address-sharing techniques.

Domain names and DNS

• A domain name is the human-readable substitute for an IP address, made of parts read right to left: the top-level (organisation or country), then the second-level name, then the host label such as "www".
• Examples of generic top levels: .com commercial, .edu education, .gov government, .org organisation; country codes: .eg Egypt, .jp Japan, .kr South Korea, .cn China.
• The DNS (Domain Name System) is the server that translates a domain name into its IP address, and can answer the reverse query.