Turning Names (URLs) Into IP Addresses

By Kerry Anders

You may wonder "How does the Internet know who you are and who everybody else is?"

The Internet routing protocols know nothing about who or where you are. All it knows is your IP address - and, of course, the IP address of those people you want to communicate with. An IP(v4) address is a 32-bit number that supports up to about 4 billion separate addresses. It is normally written in a period-separated 4-octet form (e.g. 173.194.36.104) where each octet represents a 8 bit binary number and can take any value from 0 to 255.

Now, I don't know about you, but I have trouble remembering long strings of apparently meaningless numbers, so the clever people who invented the Internet decided to use human-readable names to identify locations. These are known as URIs (Uniform Resource Identifiers), URLs (Uniform Resource Locators) and⁄or URNs (Uniform Resource Names). For the purposes of this series of articles I'll refer to them all as URLs. A URL takes the form of:

"http:⁄⁄www.google.com"

So, if everyone has an IP address but I only know their URL, how do I send them traffic? The secret is the Dynamic Name Service (DNS). This is a service, hosted on the Internet, that contains lists of URLs and their associated IP addresses. Of course, it's not quite that simple: for a start, it's not all held in one place - the URL⁄IP address lists are divided up and spread across multiple servers widely dispersed across the world.

DNS is a hierarchical service in that, if the particular DNS server you talk to doesn't know the URL⁄IP relationship you're looking for, it passes it on to another, higher level, server that either knows or knows who to ask. Eventually, if you're lucky, one server admits to knowing the information you seek and sends it to you. Your router then knows which IP address to send your traffic to and uses a whole raft of other Internet services to direct the traffic to the next router on the route to your required destination.

Unfortunately, this has to happen for every single packet of data you send! So, if DNS needed to search for the IP address for every single one of the hundreds or thousands of packets you might send out during, for instance, a Web browsing session, the system would slow to a crawl.

Hence, to speed things up, the nearest DNS server to your network remembers (caches) all your DNS requests (at least, for a while) so next time you ask, it already knows. Unfortunately, it doesn't remember for long. The nearer to your end-user device the caching DNS server is, the quicker the response to subsequent requests for the same URL is going to be.

For this reason, you want to have a DNS server somewhere on your network, and, normally, the Internet access router provides this service. All you need to tell it is where to go next if it doesn't know the address you're seeking. This is normally your ISP's DNS server.

Visit our computing and networking advice site Home and small business networking notes. This is a completely free resource site created by Kerry Anders to provide a comprehensive service to owners of home and SME networks.

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