Usually providers allow to add additional IP addresses to your server. I don't understand why. I can't imagine any common usecase which needs several IP addresses for one server.
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A few:
- Multiple SSL websites (not all browsers support SNI yet)
- Two or more applications that need to bind to a certain port
- Separation of different types of application traffic
- Multi-tenant servers, each of which have their own firewall rules
- Virtual IP, failover, CARP, VRRP, or heartbeat-type systems
- The server is acting as a router for one or more subnets
- The server is hosting one or more Virtual servers, each of which needs its own IP
- Per IP traffic accounting
- Different reverse DNS records
- Using outbound connections to services which have per IP reputation/access/rate limit control (eg. tcpwrappers, SMTP block lists)
There are countless more reasons that one might want to have more than one IP on a server, but these should at least get you started.
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Another one use case is traffic accounting per IP. – AlexD Jul 18 '11 at 04:47
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@AlexD - agreed. Go ahead and add it to my answer if you'd like. – EEAA Jul 18 '11 at 04:48
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shell hosters use multiple IPs for vanity hosts for IRC – Mike Jul 18 '11 at 04:55
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@Mike - Yes, that would fit under #3 above. – EEAA Jul 18 '11 at 04:56
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@ErikA I have added 3 more use cases, please review them. – AlexD Jul 18 '11 at 07:11
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Looks good, Alex. – EEAA Jul 18 '11 at 12:53