1. Internet Basics
February 20, 2018

2. Internet Basics

3. Internet Basics

5. Internet Basics
A significant part of what allows the Internet to work is the use of common protocols.
Every Internet-connected device “speaks” the same “language,” allowing any device to communicate with any other.
Three major protocols:
TCP (Transmission Control Protocol)
IP (Internet Protocol)
These are often mentioned together: TCP/IP (aka the Internet protocol suite)
DHCP (Dynamic Host Configuration Protocol)

6. IP Addresses
When any device is connected to the Internet, it is assigned an IP address.
An IP address identifies a device and allows it to be found by other Internet- connected devices.
This is not unique to computers and routers—printers, smart watches, security cameras, smart refrigerators, some cars, etc., all have IP addresses.
Most IP addresses currently feature four 8-bit numbers separated by periods: ##.##.##.## (e.g., ).
Since each number is 8 bits, each is between 0 and 255.
This allows for roughly 4.3 billion (232) IP addresses—and we are quickly running out.

7. IP Address Exhaustion: Why? How?
Large blocks of IP addresses are reserved for private networks:
Every IP address whose first number is 10 ( through ; 224 = million in all)
IP addresses in the range of to (slightly over 1 million)
IP addresses in the range of to (65,536)
Many Internet-connected devices are now always connected, so their IP addresses cannot be reassigned.
Every network interface on a device needs its own IP address—e.g., the 3G/LTE and wireless interfaces on a phone both need their own addresses.

8. IPv4 and IPv6
The current 32-bit address scheme (IPv4) is gradually being replaced by a 128-bit address scheme (IPv6).
IPv6 addresses have eight hexadecimal numbers and take the form ##:##:##:##:##:##:##:##.
E.g., 2001:48a8:0800:1192:0198:0110:0192:0054.
IPv6 addresses are often written without leading zeros; also, multiple consecutive zeros can be replaced by a double colon (::).
28aa:0000:0000:0000:0000:0000:0018:a5b2 can be written as 28aa::18:a5b2.

9. IP Addresses
Many (but not all) devices have permanently assigned IP addresses, known as static addresses.
Websites and servers, for instance, use static addresses: they always need to be found at the same address.
Some IP addresses are temporarily assigned; these are known as dynamic addresses.
When the device is turned off or the Internet connection is lost, the IP address is recycled and issued to another device.
In practice, these addresses do not change often, since the devices are constantly connected.

10. DHCP and DNS
Another important protocol is Dynamic Host Configuration Protocol (DHCP).
DHCP is responsible for assigning IP addresses to Internet-connected devices.
In the early days of the Internet, a single person (the network administrator) was responsible for this; the process is now automated.
DNS (Domain Name System) is central to the functionality of the Internet as we know it.

11. DHCP and DNS
Obviously, we don’t access websites using their IP addresses; we use a text- based address.
This is called a Uniform Resource Locator (URL).
When a user types a URL into their address bar, a DNS server looks up the URL and then determines the IP address to which that URL refers.
If you’ve gotten an error message that “DNS lookup failed,” this means there is no IP address that corresponds to a given URL.

12. DNS Hierarchy DNS is organized in a hierarchy as follows:
The rightmost label refers to the top-level domain: .com, .net, .org, .edu, etc.
Each label to the left specifies a subdomain of the top-level domain.
bcps.org: bcps is a subdomain of the .org top-level domain.
bcpsone.bcps.org: bcpsone is a subdomain of the bcps subdomain.
maps.google.com: maps is a subdomain of the google subdomain.
Anything after .com, .net, etc., is a subfolder.
this indicates which file we want and which path to take within the www subdomain.
Generally (but not always), this leads to a file named “index.html.”