How does a webhosting company share one IP address to many different domain names?

Quite simple. What happens is that (as a generalisation) :

1. Someone requests http://www.myname.com
2. The request is sent to the appropriate server hosting http://www.myname.com.
3. The server then registers the fact that http://www.myname.com is being requested (through the HTTP header, but that's another complex story), not http://www.mypornsite.com or some other site on the same IP number. The server thus re-directs that request to the appropriate site.

Simple explanation, but if you're techie enough and you want to get into the nitty gritty, feel free to post a request and I'll dig up info for you.

Cheers!

(edit)
I should add that I say 'as a generalisation' because WinNT and Linux/Apache do it slightly differently, and the details get messier from here on :)
(/edit)


[Edited by BC on 09-26-2000 at 07:05 AM]

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Ok, here's a variation on the question. Since it all looks like sharing IP's is going to become more common, is there a disadvantage to doing it that way? If the domain name points to a server, then gets pointed in turn to the directory, what would it matter if all domains on that server shared the same IP?

Just tell me it's a very long answer if it is one. :)

Greg Moore
http://www.akashik.net

Some of the advantages to having a dedicated IP address is the fact that you can use the IP address to reference your site without using a name. So if you wanted to point to a directory in your domain and didn't want to use your domain name for whatever reason you could do so. Without the dedicated IP address you couldn't do this. You can also use that IP address in your code if you wanted to even though it's not recommended. I can get into some more reasons but this is just a few that comes in mind. You have some control as to what you do with your site. If for some reason DNS goes down and you still need to access your site you could with the IP address tempory until the problem is resolved. Hope it helps a little.

Actually yes it does help. :) I hadn't thought of when DNS goes down, so yeah I guess an individual IP would be of benefit in most respects - it's a more 'holistic' address as it were compared to a domain name. It also opens a bit of a can of worms now I think about it. I guess linking to another site would be better by using IP numbers for a better connection, unless it's sharing an IP address....

Never mind... I'll stick with the way I do it now... :)

Thanks,

Greg Moore
http://www.akashik.net

Another item is the specter of RBL'ing one particular site on a shared IP, which would then affect all the sites using that IP. This isn't as common as Ben's scenario (that for some reason, the name would not be resolved properly), but I think Duster(?) has had experience with that type of issue. Some old, old (ancient, even! :)) browsers sometimes have a tough time, but the last web snapshots I looked at indicated that most people are moving up with the times - or at least not getting too far behind.

On the other hand, if you are a bit informed about ARIN (the org that distribute IPs) you will notice that they are no longer giving out IPs for web hosting unless an exception is made (for a name server for example)

That mean that eventually all new web sites will be on a shared IP.

As for RBL, it is not that common, since if it happens, it is because your host was too lazy to follow up the problem and fix it. If this is the case... why are you with them! :)

People usually mention the fact that you can access your site before your domain is pointing to the new host... However, with minimal knowledge, the host can create a pointer to your web site and will allow you to see it as:
http://www.yourhost.com/http/www.yourdomain.com or something similar (that was what we do).

Anyway, in teh near future there will only be that kind of hosting available... untill IPv6 comes out (and even then... will it be available?)

There is no major drawback in my opinion.

RBLing? <blink.....blink>

what the heck is that?? :)

Greg Moore
http://www.akashik.net

Sorry! Realtime Blackhole List, where email goes to die. Spam-city, generally, but if you're tied in to one of the IPs (or a block of IPs) that the spammers use, and they get put on the RBL, you might get caught up in the RBL yourself. Like I said, it isn't really that common, but it remains a possibility to keep in mind.

As far as ARIN's policy goes - they've had that (unwritten) policy for some time. Blocks of IPs are still being given out to ISPs/hosts, etc., especially if the requestor is a big gorilla. I disagree with Felix - IMO, dedicated IP addressing is not going away anytime soon, even given the wait for IPv6.

For a lot of people, though, IPless hosting is no problem unless they absolutely require anonymous FTP, or nameservers, or SSL (although this can be worked around), for instance. Most personal sites? Point 'em to a single IP and have at it. :)

Originally posted by cahostnet
If for some reason DNS goes down and you still need to access your site you could with the IP address tempory until the problem is resolved.Of course, even if you are sharing an IP you still have an IP. So if DNS were to go down as long as you knew which IP you're sharing you could get there; you'd then have to specify which directory your particular site is in. A little messier, but still possible.

Originally posted by Annette

As far as ARIN's policy goes - they've had that (unwritten) policy for some time. Blocks of IPs are still being given out to ISPs/hosts, etc., especially if the requestor is a big gorilla. I disagree with Felix - IMO, dedicated IP addressing is not going away anytime soon, even given the wait for IPv6.


coming from http://arin.net/announcements/policy_changes.html


Use of Name-based Virtual Webhosting
The name-based system of virtual webhosting used by many ISPs today allows multiple domains to be hosted by a single IP address. While some organizations use IP-based webhosting to, in part, justify their requests for IP space, ARIN will no longer accept IP-based hosting as justification for an allocation unless an exception is warranted. The ARIN Instructions for Using Name-based Virtual Webhosting may be a helpful tool in setting up, converting to, and using name-based hosting.


That is found under ANNOUNCEMENTS
Policy Changes result from ARIN's Public Policy & Members Meetings

:) that is quite a written policy if you ask me Anette :)

<edited> But you are right... you need a dedicated IP for SSL and anonymous FTP... for now </edited>

OK, let me clarify, since obviously that wasn't clear enough: they have had this policy as an unwritten policy for some time. Just about everybody knows about the announcement they put on the site, and I presumed that was what you referring to when you mentioned the policy. It is not new. They have simply made it more public now to try and discourage massive block buying. It does not mean that dedicated IPs are no longer available, nor does it mean dedicated IP addressing is going away, period. This issue has been discussed at length in several forums, and at this point, significant changes have yet to be seen.

I just find it strange for an organization to post a policy they will not respect?

I know they don't want any single company out there to register tons of IPs but I am not as confident as you are that they will still handle them out to the hosting companies.

Anyway, that's not the point of what I said... the point was that it is not essential to the majority of web sites, since most display text files, pictures, have some cgi scripts but that's all. The dedicated IP is not really usefull then.

As for if the DNs break... your visitors will not have the IP to your site, so you can access it, but not your visitors (you can always access it if its name based, through a redirection url)

That's it :) no need to argue over this :)

Well, remember - this is ARIN. Like InterNIC, the rules are apparently not as firm as they might seem. :)

As far as IP addressing - the other side of this is the huge companies that pay ARIN's way. They have certain ideas about how they want things done, and this being the US, the big companies usually rule. RIPE et al have done a much better job at IP reclaiming than ARIN, apparently.

But absolutely: most people don't need a dedicated IP. Heck, a lot of them don't even know what an IP address is (or a CPU, or RAM, or...). Part of the issue is that these very people think they must have one, even if what's on the site is pictures of their cat Fluffy.

As someone with first hand experience with RBL (see http://mail-abuse.org/rbl), I want to correct a misconception here. It had nothing to do with a shared IP. A block of IP addresses from my previous host (Media 3) was listed in the RBL. MAPS removed mine promptly after I communicated with them.

As it turns out, Media 3 is rated the top spam haven by the Spamhaus Project (http://spamhaus.org)

[Edited by Duster on 09-26-2000 at 06:36 PM]

Thanks, Duster. I couldn't remember if you were on a share or just lived on the same block.

You're welcome. You've already pointed out the valid reasons for dedicated IPs. For the benefit of others, I'll add that there is a lot of misinformation about shared IP addresses out there. Some people will tell you that major search engines won't list your site, or list it high up, if you have a shared IP address. This is not true.

Some will point out that your site can be banned if it shares an IP address with an undesirable site. This is true, though it is not limited to shared IPs. A site can be banned if it is in a block of IP addresses that are banned (either by search engines or sites blocking spammers). The shared IP address is then a moot point. Pretty much anything that can happen to you with a shared IP address can happen with a dedicated one.

One thing the IP issue makes clear is the importance of choosing a host that is not a pariah to the Internet. If you choose a spam have like Media 3, Interland, or others, you may find yourself cut off from communication with a substantial portion of the Internet regardless of your IP address situation.