What is HTML?
Hypertext mark-up language is a "computer language" used to structure web pages.
What are the tags you need to know for the exam?
HTML uses tags to structure a webpage. For the exam you will need to know these tags:
<html> = beginning of a html page
<head> = the heading and title of a page
<body> = the main body of text of a page
<em> = italic
<strong> = bold
<p> = paragraph
<hr> = horizontal line across the page
<br> = break in the text
<a href> = hyperlink
<img> = image
What is CSS?
Cascading Style Sheets. This is used to style web pages. CSS is different to HTML as it separates document content from document presentation, including elements such as the layout, colours, and fonts.
CSS is split up using selectors e.g.
p {color : Blue;}
Sunday, 10 October 2010
Sunday, 3 October 2010
The Structure of the Internet
The internet is a Global network of networks that all overlap each other like a mesh. This is so that if we lose a few links it wont cause a big problem.
Information is moved around the internet via packets. These packets contain key things: the sender's IP address the receiver's IP address and a number that tells the sequence that it goes in.
The Intranet is a small network intended for a company or school. It can host private websites and can be used for communication within the company.
The Extranet is a network that allows controlled access from the outside of a specific business. It's like an extension of the Intranet.
Information on the internet is addressed as an IP address (Internet Protocol address) this is a numerical label specific to a certain network. It would look a bit like this: 208.77.188.166
Computers have domain names which are labels for IP addresses (human readable). A DNS (Domain name server) would translate domain names into IP addresses. An Internet registrar allocates a block of ip addresses to a certain region of the world.
A client-server model:
The most common model for distributing a system is the client-server model. The model is fairly simple to explain and use.
The name of the model is quite descriptive. In your distributed system you have one or more servers. These servers provide services to other parts of the system, called clients. When a server is started it first opens up a particular port through which clients can access it. It then sits down and waits until somebody (the client) attempts to connect to it. When that happens, the server and client exchange some messages and ultimately of the two close the connection. This connection takes place using so-called sockets.
The Server
The simplest version of such a server is non-threaded. That means that multiple connections are handled sequentially, in other words: clients have to queue up and they are handled one by one. That's fine if connections last only very short and if there are not too many clients connecting at the same time, however if you have many clients connecting at the same time or long-lasting connections you have to handle connections in parallel. You can handle multiple connections in parallel using threads. Each time a connection is established a new thread is created and the connection is handled by that new thread. The server thread then continues accepting new connections. Because creating threads is an expensive process (in terms of CPU cycles) threads are usually kept in a "pool". When a thread finished its job, it is kept alive until a new request has arrived it can handle.
The Client
For the client to connect it has to know the server's IP or hostname and the port to connect to. Once the connection is established the client and server can exchange messages. Depending on the distributed system a client may connect to multiple servers. One server to access the database, one for file services
The name of the model is quite descriptive. In your distributed system you have one or more servers. These servers provide services to other parts of the system, called clients. When a server is started it first opens up a particular port through which clients can access it. It then sits down and waits until somebody (the client) attempts to connect to it. When that happens, the server and client exchange some messages and ultimately of the two close the connection. This connection takes place using so-called sockets.
The Server
The simplest version of such a server is non-threaded. That means that multiple connections are handled sequentially, in other words: clients have to queue up and they are handled one by one. That's fine if connections last only very short and if there are not too many clients connecting at the same time, however if you have many clients connecting at the same time or long-lasting connections you have to handle connections in parallel. You can handle multiple connections in parallel using threads. Each time a connection is established a new thread is created and the connection is handled by that new thread. The server thread then continues accepting new connections. Because creating threads is an expensive process (in terms of CPU cycles) threads are usually kept in a "pool". When a thread finished its job, it is kept alive until a new request has arrived it can handle.
The Client
For the client to connect it has to know the server's IP or hostname and the port to connect to. Once the connection is established the client and server can exchange messages. Depending on the distributed system a client may connect to multiple servers. One server to access the database, one for file services
a protocol is the special set of rules that end points in a telecommunication connection use when they communicate.
On the Internet, there are the TCP/IP protocols, consisting of:
- Transmission Control Protocol (TCP), which uses a set of rules to exchange messages with other Internet points at the information packet level
- Internet Protocol (IP), which uses a set of rules to send and receive messages at the Internet address level
- Additional protocols that include the Hypertext Transfer Protocol(HTTP) and File Transfer Protocol (FTP), each with defined sets of rules to use with corresponding programs elsewhere on the Internet
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