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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:

  1. Multiple SSL websites (not all browsers support SNI yet)
  2. Two or more applications that need to bind to a certain port
  3. Separation of different types of application traffic
  4. Multi-tenant servers, each of which have their own firewall rules
  5. Virtual IP, failover, CARP, VRRP, or heartbeat-type systems
  6. The server is acting as a router for one or more subnets
  7. The server is hosting one or more Virtual servers, each of which needs its own IP
  8. Per IP traffic accounting
  9. Different reverse DNS records
  10. 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 4:47
@AlexD - agreed. Go ahead and add it to my answer if you'd like. – ErikA Jul 18 '11 at 4:48
shell hosters use multiple IPs for vanity hosts for IRC – Mike Jul 18 '11 at 4:55
@Mike - Yes, that would fit under #3 above. – ErikA Jul 18 '11 at 4:56
@ErikA I have added 3 more use cases, please review them. – AlexD Jul 18 '11 at 7:11
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