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Why would I need more than 1 IP on a web server? Some people buy 4 IPs!

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Multiple SSL certificates.

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1 (though it isn't that common) – grawity Feb 25 '10 at 15:23

Having a single IP allows you to use virtual hosts, but you will only have one default site for that IP. If a request doesn't match a host header then this default site is served, which might not be a suitable option. For example, in a shared hosting environment, if for some reason a client's website is not operating correctly (perhaps a misconfiguration) then the hosting company's default site (often theirs) will display.

Having said all of that, multiple SSL certificates is the much more common reason.

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If you want multiple web sites to have their own IP addresses, or if you want one IP for web traffic and others for other purposes such as remote administration.

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As mentioned multiple SSL certificates is the most common reason, but there are a couple of other reasons you will frequently see.

Sometimes people run dns servers on the same machine as their web server. In this case, you need two ip addresses for two different name servers in order to comply with RFCs. Of course the RFC was designed to give dns redundancy so name resolution for a single domain doesn't rely on a single machine, which this isn't providing and instead causes more ip addresses to be used than necessary.

FTP doesn't work by domain names like http does, so if you want multiple ftp servers on a single machine then this is another reason to get an extra ip address.

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You can only have an SSL certificate listening to a single IP, so to have 2 SSL certificates- you need 2 ip addresses. You might also want to route traffic to certain IP's differently.

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