Re: [SLUG] Sites on IP

From: Ian C. Blenke (
Date: Tue Apr 11 2006 - 15:11:19 EDT

Paul M Foster wrote:

> Two questions:
> 1. Since many sites can share the same IP, is there any way to tell
> all the websites which share an IP?

Since HTTP 1.1, "Named virtual hosting" allows clients to send a "Host:
" header in the http request to ask for a specific website.

In DNS, many records point to the same IP address. Clients connect to
that IP address and send the "Host:" header for the site they wish to

The reverse PTR record for any given IP doesn't need to match anything
at all (though it will probably reflect your provider's hostname for
their own purposes).

A virtual hosting provider uses this fact to allow many websites to be
hosted on a single IP address. When a client web browser connects, it
tells the web server which site it wants content for.

Older HTTP 1.0 clients do NOT send the "Host:" header, and thus get the
"default" website content (usually the web hosting companies' default

You might complain to your provider that older HTTP 1.0 clients cannot
see your webpage. If they are contractually obligated to some service
level and don't mention this limitation, you might convince them to give
you your own IP for your web hosting.

There is no way that I am aware of to enumerate the available "named
virtual hosts" on a given HTTP 1.1 connection. The best you can hope for
is to whois and reverse lookup the netblocks to find some mention of the
domains hosted by your provider. You might also try googling the IP.

Aside from that, there really isn't much you can do outside of looking
at your provider's HTTP server configs or directly sniffing the packets
coming in and out of the server.

> 2. Assuming you have a "reseller account" with a hosting company,
> where they give you two IPs for your sites, is it possible or probable
> that they have also given someone else those same IPs? Is this common
> practice or not?

It really all depends on how the service is implemented. If you don't
have your own virtual image that you yourself administer for those IPs,
I'd wager that many other domains probably share those IPs. With reverse
proxies and other trickery, literally anything is possible.

- Ian C. Blenke <>

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