Computer Science Lecture
October 25, 2010
Network: A group of computers that can talk to each other.
- There is a central computer in one city that communicates with millions of other
ones. (like the star formation from MIT)
- If the central computer fails, the whole network fails.
- The best layout for a network is the Spider Web. Star and Connect Formation.
Every computer is connected to every other, back and forth
- A global system of interconnectd computer networks that use the standardized
Internet Protocol Suite (TCP/IP) to serve billions of users worldwide.
How Does the Internet Work?
- Uses TCP/IP
- A standard protocol way of communicating.
- The ideas behind this protocol were funded by the ARPA of the US DoD around
1969. Thus the internet was originally called the ARPANET
- Opposite of your home phone where you get a direct line that is only between you
and the person you are talking to. The Internet tries multiple lines, it doesn’t make
sure the line is clear first, it just tries and fails or succeeds at random lines.
- Circuit Switching: Telephone has one direct route to a person, blocking all other
lines and has all of the information
- The Internet breaks an email message, a web page, everything into things called
packets which are just like pages of a manuscript. All of the packets are sent down
- Packet: A small group of bytes consisting of a header (tells where it is going:
destination and where it came from: source) and the body. (Often 64 bytes for
header and 512 bytes for the body)
- Take a large chunk of data (email, webpage) and breaks it into packets.
- Sends the packets out to the Internet.
- At the receiving end:
- Detects lost packets with errors because of network congestion and
rearranges them and puts them all together.
IP (Internet Protocol)
- Like a GPS
- Picks a route for a packet, stopping at routers which pick the next best
machine/network to send the package to.
- If a communication line is down or broken, sends the package back to TCP and TCP
sends it again to try a different route.
- Needs to be able to identify all the machines on the Internet , thus each machine
has its own unique address.
- Uses IP Addresses (every machine is unique IP Address)
- IP Addresses consists of 4 numbers with dots between them. Each number ranges
from 0 to 255. Sample: 129.100.23.003 - IP Addresses are