Understanding DNS requirements
An important part of upgrading your Exchange organization is configuring your DNS namespaces correctly. During the time when your organization is using multiple versions of Exchange, you'll need to add a legacy DNS namespace that will be associated with your legacy Exchange organization.
Configuration of a legacy DNS host name is only required if you plan to run both Exchange 2010 and either Exchange 2003 or Exchange 2007 for a period of time. If you have a small number of mailboxes and will be moving them from Exchange 2003 or Exchange 2007 to Exchange 2010 over a scheduled downtime, you don't need to configure a legacy host name.
Your existing Exchange organization has an existing DNS namespace that allows your users to access Exchange services such as Outlook Web App and Exchange ActiveSync. This DNS namespace is likely in the format of mail.contoso.com. As part of the upgrade process, you'll need to configure a second DNS namespace in the format of mail.legacy.contoso.com.
As a best practice, we recommend that you use split DNS for the Exchange hostnames that are used by clients. Split DNS refers to a DNS server configuration in which internal DNS servers return an internal IP address for a hostname and external (Internet-facing) DNS servers return a public IP address for the same hostname. Because using split DNS uses the same hostnames internally and externally, this strategy enables you to minimize the number of hostnames you'll need.
The steps to perform this configuration will vary with your firewall and Internet Service provider. Contact your network administrator for more information about how to perform this configuration.
After you have installed Exchange 2010 and configured the Exchange 2010 Client Access server, you'll need to configure your primary DNS namespace of mail.contoso.com to point to your Exchange 2010 Client Access server and then configure mail.legacy.contoso.com to point to your Exchange 2003 or Exchange 2007 infrastructure.
After you decommission the last Exchange 2003 front-end server or the last Exchange 2007 Client Access server in your organization, you can also remove the legacy DNS namespace that you created for the upgrade process.