All domains that have been registered initially through 1&1 will use the 1&1 name servers (DNS). For domains that are transferred to 1&1, the DNS settings can usually be chosen when initiating the transfer and therefore will vary. For more information about what DNS is, please read the Explanation of DNS Settings.
Depending on what type of 1&1 package the domain is being added to (or ordered with), the DNS settings may slightly vary but all domains will use the 1&1 name servers (DNS).
All domains registered with 1&1 will use the 1&1 name servers (DNS), will use 1&1 mail servers and if applicable, point to the correct IP address if the domain is added to a package that offers web hosting (web space).
The DNS settings can always be altered after the domain has been registered to point to a different IP address, mail servers, or even change the DNS altogether (DNS Transfer or CNAME). Please reference the DNS Settings section of the 1&1 Help Center for more information about changing DNS settings.
Domains that are being transferred to 1&1 from another company will prompt you to configure your DNS settings when initiating the transfer but can always be changed at a later time if necessary. For more help on transferring a domain to 1&1, please reference one of the below articles: