Why IP Reservation Matters For Cloud

Cloud deployments typically rely on abstracting away as many details as possible. So why would you want to assign something to a very specific IP address?

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One of the Azure features Microsoft announced during TechEd North America this week was IP reservation: the ability to tie your applications to specific IP addresses.

"In Azure, when you do a deployment, we give you a public IP from our dynamic pool of addresses," Azure program manager Narayan Annamalai explained during a press briefing on the new service. "The IP sticks to that service as long as it runs, but if you delete and redeploy, a new IP is assigned."

"Mostly access to those services is DNS-based and not a problem," Annamalai noted. In other words, usually users will be directed to a specific domain name rather than a specific IP address, and as long as the domain is mapped to the current IP, changes aren't a big problem. However, in some contexts companies do want to maintain a specific address. For instance, a monitoring service tracking the health of applications might want to inspect IPs, not domains.

Azure now offers two options covering that area. IP reservations for VIPs allow you to reserve public IP addresses for applications. Each Azure subscription can have five free of charge, and add another five (for additional cost) if you wish.

Instance-level public IPs allow you to assigned IP addresses to individual virtual machines rather than applications. "This feature will enable scenarios such as running FTP servers in Azure and monitoring virtual machines directly using their IPs," Microsoft's launch notes suggest. That option is currently in preview, and you can have a maximum of two addresses.

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    "as long as the domain is mapped to the current IP, changes aren’t a big problem".

    Whoever wrote this has obviously never had to change the ip of a major domain name before. DNS caching issues mean the risk of down time for a subset of users is high, particularly, unfortunately, for Hellstra users or users with Micro$oft Exchange. This is why ip address reservation is absolutely critical for front end services.

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