School of Computing. Dublin City University.
Online coding site: Ancient Brain
Each site could keep a list of all sites on the network
(or have access to such a list).
BITNET kept such a list, of thousands of international sites.
ARPANET did this at first, too.
The Internet has long since grown too big to keep a list of all sites anywhere, let alone have one at each site.
This means sites do not know a priori if an email address or web site name is valid. They must make a query and find out.
You don't clutter up your site with info about other sites, 90 percent of which you will never actually need to know about. You don't have huge amount of Internet traffic being announcements of new sites.
Say the University of MIT is responsible for all addresses:
*.mit.eduIt is assigned a segment of the numerical address space, like this (not exactly this):
106.132.*.*and can assign and reassign names to this as it likes. It doesn't need permission to add new machines to the Internet, nor does it need to publicise those machines. Other sites find out about these machines if and when they need to (which may be never).
When given the name of a machine:
jimmy6.chemistry.mit.eduthey talk to the DNS server for .edu, which gives them the name of the DNS server for .mit.edu, which gives them the real address of the site (or says "invalid site").
Each subzone can build and rebuild its own topology as it pleases. Don't even need to report how much of that address space is used.
DNS so important / done so often that there are duplicate DNS servers, caches of remote information, etc. (without going as far as the complete cache of all sites on the net).
DNS uses UDP (not TCP) for address lookups.
The root nameservers of the world, as at 2006.
13 logical servers, most distributed geographically using anycast.
There was at that time a single root nameserver in Ireland, run by INEX in Dublin. Part of the J group. A nameserver for .com and .net.
CA DNS server: 184.108.40.206 DCU DNS server: 220.127.116.11
Multiple names for the same host.$ nslookup www.computing.dcu.ie www.computing.dcu.ie canonical name = corno.computing.dcu.ie. Name: corno.computing.dcu.ie Address: 18.104.22.168
Example of shared hostingThe below is about the old hoster for my genealogy site. I have to change this since I have moved hoster. But the point still holds.
My genealogy site humphrysfamilytree.com is hosted with a US company. See lookup. The hosting is not cheap or unusual. Just a normal hosting service.
My site often gets healthy (I thought) traffic of around 100,000 to 150,000 page views per month. And yet I was startled to discover that it is sharing the same IP address with 762 other sites. Lookup from here.
Thinking about it, my site traffic works out at only 2 to 4 page views per minute, which is nothing for a powerful modern server. Lots of room for shared hosting.
The note about "adult" sites indicates a few things about modern hosting:
- If "adult" content is legal, the hosting service may well have customers with such content. This could easily be on the same IP as your site.
- If web filtering software filters by IP, it may block many legitimate sites.
- The hosting company could never police all its content, "adult" or not. No one could run a hosting service that had to police content.
- Which is not to say that hosting services don't respond to specific requests from police/courts about illegal content. They do. But they cannot police content in advance. The numbers are too big.