Before we go any further, let's define (and explain) some terms.
| IP Address | An Internet Protocol address. The "phone number". A 32-bit value often represented in "quad" or "dotted decimal" format - four numbers between 1 and 255 separated by dots ('.'). This is the actual number that has to be used to reach a remote site. |
| URL | Uniform Resource Locator. The "friend from work". The name the DNS must look up for you. Your Internet client (your browser, your FTP program, your telnet program, whatever) has to look up this name with the DNS and get back an IP to connect with. |
| DNS | The Domain Name System. Your "directory assistance". Your ISP provides addresses for one or two name servers - your "directory assistance operators". These become your first contacts with the DNS, and from this point your queries will go out into the world, until they've found the "number" you're looking for. |
| DNS Query | The process of submitting a URL to the DNS and receiving an IP in return. The "call to directory assistance". |
| Reverse DNS (Inverse DNS Mapping) | Deals with the curious case where you have an IP but want to find out the identity of the remote site. Reverse DNS normally works on all IPs, operative word "normally". |
| ISP | Internet Service Provider - the company that connects with your modem so you can get out onto the Internet. This ISP will normally provide you with IP addresses to one or more local DNS servers so your computer can find remote sites. |
| Hosts Cache | Your "phone list". A file on disk which stores pairs of URLs and IPs to save you time when connected to the Internet. Your TCP layer will always check this file first to see if it can find the URL you want to reach there instead. Only if it is not in the hosts cache will the TCP layer go out onto the net and query the DNS. |
| TCP | Transmission Control Protocol. The Internet equivalent of the telephone system. When the other person answers, you have a connection. |
| IP | Internet Protocol. The basic building block of Internet communications. Runs under other higher level protocols such as TCP. |
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