Turning Names (URLs) Into IP Addresses
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. 126.96.36.199) 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:
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.
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