Instant Messaging Domains

An Instant Messaging Domain is a DNS name that identifies user accounts. Just as an e-mail address contains a specific e-mail domain after the @ sign, an extended Instant Messaging address contains a specific Instant Messaging domain after the @ sign.

It is recommended that Instant Messaging domains have a one-to-one correspondence with e-mail domains. For example, users with an e-mail address "@microsoft.com" would be hosted in the Instant Messaging domain "@im.microsoft.com." Relating IM domains one-to-one to e-mail domains allows the use of existing SMTP addresses as Instant Messaging addresses. In cases where a user has more than one e-mail address defined, the domain of the primary e-mail address should be used. This is also usually the externally visible e-mail address that those outside the organization use to communicate with this user.

Although it is not required, it is recommended that the Instant Messaging domain for a given e-mail domain be derived by adding the prefix "im." to the e-mail domain. Adding the prefix "im." to the e-mail domain lets users in remote locations (those without access to external DNS names) infer the extended IM address "@im.microsoft.com" from the standard SMTP address "@microsoft.com," which allows them to communicate with users in that domain.

Some companies may have only one Instant Messaging domain (for example, im.microsoft.com). Large companies that partition their externally visible e-mail domains by country will likely have several Instant Messaging domains (for example, im.us.microsoft.com for users whose e-mail address is "@us.microsoft.com" or im.jp.microsoft.com for users whose e-mail address is "@jp.microsoft.com,"). Most companies currently use just one externally visible e-mail domain, and therefore have just one Instant Messaging domain.

Since an Instant Messaging router can answer queries arriving at only one Instant Messaging domain, there should be at least one Instant Messaging router for each e-mail domain. The Instant Messaging routers for an Instant Messaging domain receive all Instant Messaging traffic addressed to that domain and forward or redirect that traffic to the respective Instant Messaging home servers hosting the user accounts.

Each Instant Messaging domain name require a DNS entry to point the domain name to the IP addresses of the Instant Messaging routers for that domain. If allowing communication from outside the company (for example, from the Internet) to users within this domain is desired, this DNS entry also needs to be externally visible and instead must point to the reverse proxy for that Instant Messaging domain.

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