Network addressing

Network addressing

For any communication to succeed, the message must have an unambiguous destination, just as every telephone in a country needs a unique number.

On a network, you will encounter various forms of addressing to ensure traffic gets where it should without confusion or error.

Addressing

Network address

Every node on a network must have a unique identifier so the file server knows who is requesting information, and who is to be sent information. This unique identifier is hardwired into the NIC. When a workstation requests data, it sends its network address with the request so the server knows which node to reply to.

IP addresses

Since the Internet is just one massive network, each connection on the Internet also needs a unique identifier for communication purposes. This is an Internet Protocol (IP) address.

Some IP addresses are granted just for the duration of a connection to the Internet. These are dynamic IP addresses. Typically, an Internet Service Provider has reserved a big enough pool of IP addresses to allocate one IP address to each of their modems. When users dial up the ISP, they are each assigned an IP address for the duration of their calls. This means that every time you connect to the Internet by dialup modem, you will probably get a different IP address.

A subnet mask (e.g. 255.255.240.000) is sometimes used to get multiple local IP addresses from a single IP address. An organisation with a single IP address can use submasks to create a large number of  'subnet addresses' for its many workstations. A simple analogy would be telephoning a large organisation's single phone number, and asking for "Extension 202 please".

Other devices, such as web servers and cable modems, are connected to the Internet permanently. These devices are allocated a permanent IP address, called a static IP address because it does not change. Because hackers can target computers by their permanent IP address, it is important that cable modem users install a firewall.

See the following section, "How Packets are Handled on a Network" to see how the IP address works.

Uniform Resource Locators (URLs)

URLs are human-readable addresses of internet resources such as web pages (e.g. http://www.mckinnonsc.vic.edu.au) and FTP sites (e.g. ftp://ftp.microsoft.com). URLs are easy for a human to remember and understand, but they are no good for computers which work best with numbers. When you enter a URL, the address is looked up by a DNS (Domain Name Server) and translated into a numeric IP (Internet Protocol) address. IP addresses consist of 4 'octets' of numbers e.g. 10.77.91.19 or 209.112.113.56 where each number between the dots can be from 0 to 255. It is the IP address that is actually used to locate any Internet location.

Did You Know.

The Domain Name system is a distributed database spread throughout the world on thousands of machines, yet it behaves like a single, integrated database! It contains information on every internet  location on the planet.,

A URL dissected

http://www.mckinnonsc.vic.edu.au/events/2002/hmf/index.htm

Http://  Advises the web browser to use the Hypertext Transfer Protocol for this data transfer. HTTP defines the commands and procedures passed between web browsers and web servers. If you were downloading files, you'd use ftp:// to signal you wanted to use the File Transfer Protocol.

www   Indicates the file is on the World Wide Web. Many Internet domains do not require the 'www' to be used at the beginning of their name.

mckinnonsc.vic.edu.au - the name of the domain. The ".au" is the standard country code for Australia. URLs with no country code are usually in the United States. "edu" means the site is an educational establishment. Other "top level domains" include gov (government sites) and mil (military sites). The most common domain types are com (commercial site), net (networks) and org (organisations), but they often mean little. Domains registered in the .au domain have strict rules governing names. For example, applicants must provide evidence that they own a business with the requested name. Domains registered in America have no such requirements.

/events/2002/hmf/index.htm

After the domain name finishes, the rest of the URL describes the location of the requested file on the web server's hard disk. It is very much like the description of a pathname on your own hard disk. The example means "Enter the events folder,  then enter the 2002 folder, then enter the hmf folder. In there, retrieve the file called index.htm. If no filename is given, the web server will automatically try to find a file called index.htm, index.html or default.htm and load it. The opening page of nearly all websites is given one of these names. If a requested file is not found, the server returns a "404 - file not found" error.

Email addresses

An email address like fred@somewhere.com is easier to decode. Before the "@" symbol is the username of the emailbox owner. After the "@" is the domain that hosts the emailbox. Senders of unsolicited email (spam) use software 'robots' to search webpages and newsgroups for email addresses: the robots look for text containing an "@" and at least one dot. To counter this, many people "munge" or alter their email address to camouflage them. For example, fred AT somewhere.com or fred@somewhereNOSPAM.com. Humans can see the obvious munging and fix the address, but 'spambots' either don't recognise it as an email address or the spammers could not be bothered manually fixing munged addresses.  A popular way of disguising an email address is to create it in a paint program as text in a picture. That way, people can read it, but spammers can't harvest it.

MAC addresses

The Media Access Control (MAC) address uniquely identifies each node of a network. MAC addresses are often referred to when setting up wireless nodes.  It has nothing to do with Macintosh computers!