Part I of Cisco Networking Simplified

Cisco Press

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  • E-mail clients that are installed on individual machines are in wide use today. The most popular are Microsoft Outlook/Outlook Express. E-mail clients allow for the creation, distribution, retrieval, and storage of e-mails (as well as some other useful features). These types of clients were originally designed so that e-mails to and from an account could be accessed from a single machine.

  • E-mail clients physically move the e-mail from the e-mail server to your PC's hard drive. After the e-mail is downloaded, it no longer exists in the e-mail provider's network. The e-mail exists in your e-mail client program (on the PC's hard drive) until you delete it.

  • Web-based e-mail tools, such as Google Mail, allow users to access their e-mail from any machine connected to the Internet. Users log in to the website with their registered name and password. Then they are given access to a web-based e-mail client that has all the basic abilities of e-mail clients, such as the ability to create, send, and receive e-mails. Many have more advanced features, such as the ability to send and receive file attachments and create and use address books.

  • Web-based e-mail tools differ from e-mail clients in that the e-mail is not downloaded to your PC's hard drive. It exists only on the e-mail provider's network until you delete it. Some people use a combination of web-based e-mail and e-mail clients. For example, you may use the web-based e-mail tool to access your e-mail when you are away from home and not using your home PC. When you are at home, you could then use your e-mail client.

What's Up with the @ Sign?

All e-mail addresses are made up of two parts: a recipient part and a domain name. An @ symbol separates the two parts to denote that a recipient is unique within a domain name. The domain name is usually the name of your ISP (or your company if you have e-mail there), and, like a website, an e-mail domain has an associated IP address. This allows (actually, requires) the use of a DNS server to translate the domain name portion of an e-mail address to the IP address of the server where the e-mail account resides.

The recipient part is the chosen identifier that you are known by within the e-mail domain. There are a lot of possibilities for choosing the recipient. Here are a few popular styles:







Personalized license plate


Other obscure reference


When picking an e-mail address, remember that sometimes you'll have to verbally tell someone your e-mail address, so "X3UT67B" is inadvisable.

Sending E-Mails

E-mails are distributed using a (OSI Layer 7) protocol called Simple Mail Transfer Protocol (SMTP). SMTP normally operates on powerful computers dedicated to e-mail distribution, called SMTP servers. When you create and send an e-mail, your e-mail client sends the file to the SMTP server. The server pulls out the addresses from the message. (You can send e-mails to multiple recipients.) For each domain name, the SMTP server must send a message to a DNS to get the IP address of each recipient's e-mail server. If the recipient is on the same server as you (that is, if you send an e-mail to someone with the same domain name), this step is unnecessary.

After your SMTP server knows the IP address of the recipient's server, your SMTP server transfers the e-mail message to the recipient's SMTP server. If there are multiple recipients in different e-mail domains, a separate copy of the e-mail is transferred to each recipient's SMTP server. According to the name of the protocol, this is all pretty simple.

Receiving E-Mails

E-mail is often received via a different server than the one that sends e-mail. The type of server depends on which type of e-mail tool you use. For those using an e-mail client, your e-mail is probably delivered to you via the most common method, Post Office Protocol 3 (POP3) server. (We have no idea what happened to the first two.) The POP3 server receives all its e-mails from SMTP servers and sorts them into file spaces dedicated to each user (much the same way mail is put into post office boxes at a local post office—thus the name).

When you open your e-mail client, it contacts the POP3 server to request all the new e-mails. The e-mails are then transferred to your PC, and in most cases the e-mails are erased from the POP3 server.

Another common method (or protocol) for mail retrieval is an Internet Mail Access Protocol (IMAP) server. This is the protocol normally used by web-based e-mail clients, and corporate e-mail systems such as Microsoft Exchange. The IMAP server receives and sorts e-mail in much the same way as a POP3 server. Unlike POP3, however, IMAP does not transfer the e-mails to the machine of the account holder; instead, it keeps e-mail on the server. This allows users to connect to use their e-mail account from multiple machines. IMAP also allows for server-side filtering, a method of presorting e-mail based on rules before it even gets to your PC. It's kind of like having a friendly postal worker who sorts all your bills to the top and magazines to the bottom.

Two main issues with IMAP servers are storage space and working offline. Most Internet e-mail services put a limit on the amount of storage each subscriber gets (some charge extra for additional storage space). In addition, these services often limit the file size of attachments (such as photos). The other issue is the ability to work offline or when not connected to the Internet. One solution is called caching, which temporarily places the subscriber's e-mail information on whatever PC he or she wants to work offline with. When the user reconnects, any e-mails created while offline are sent, and any new incoming e-mails can be viewed.

Web Browsing

Browsing web pages on the Internet is another common network application. Browsers run on a computer and allow a viewer to see website content. Website content resides on a server, a powerful computer with a lot of disk space and lots of computing cycles. The protocol that allows browsers and servers to communicate is HTTP.

What Are HTTP and HTML, and What Do They Do?

You might have noticed that many Internet sites include the letters HTTP in the site address that appears in the address line of your web browser. HTTP (another OSI Layer 7 protocol) defines the rules for transferring information, files, and multimedia on web pages. Hypertext Markup Language (HTML) is the language used within HTTP. HTML is actually a fairly simple, easy-to-learn computer language that embeds symbols into a text file to specify visual or functional characteristics such as font size, page boundaries, and application usages (such as launching an e-mail tool when a user clicks certain links).

When the developer of an HTTP file (or web page) wants to allow for a jump to a different place on the page, or even a jump to a new page, he or she simply places the appropriate symbols into the file. People viewing the page just see the link, which is most commonly specified with blue, underlined text. The ease of jumping from site to site (called web surfing) is one of the reasons for the proliferation of websites on, and growth of, the Internet.

Several free and commercial tools allow you to create a web page using HTML without having to know all the rules.

One of the issues with HTML is that it is fairly limited as far as what it can do given that it works only on text and still pictures. To achieve some of the really cool moving graphics and web page features, other tools such as Flash, XML, JavaScript, or other scripting languages are needed.

Copyright © 2007 Pearson Education. All rights reserved.

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