DNS basics

Domain names, like contoso.com, are managed by using a worldwide system of domain registrars and databases. The Domain Name System (DNS) provides a mapping between human-readable computer hostnames and the IP addresses used by networking equipment. An understanding of DNS and domain registrar basics will help admins manage domains in Office 365.

What do you want to do?

Learn about domain names

Domain names are used in URLs and email addresses, and they have different levels. For example, mail.contoso.com is a domain name with the following three levels:

  • .com is the top-level domain
  • contoso is the second-level domain
  • mail is the third-level domain

Why use a third-level domain? You might want to have different domain names for marketing or a blog. For example, blog.contoso.com.In Office 365, third-level domains are available with Office 365 Enterprise and Office 365 Midsize Business, but not with Office 365 Small Business.

Understand DNS record types

DNS records are used to direct traffic to and from your domain. The following table describes frequently used DNS records and how they’re used with Office 365.

NS (name server) record Identifies the name servers that are the “authoritative name servers” for a domain. When you change these servers, you change where your DNS records are managed and where the DNS system looks for information about mail servers and so on.
A record (address record) Associates a domain name with an IP address.
CNAME (alias or canonical) record Redirects one domain to another in the DNS system. When a name server looks up a domain and finds that it has a CNAME record, the server replaces the first domain name with the CNAME, and then looks up the new name.
MX (mail exchanger) record Points to where your email should be sent. It also has a priority field so that you can send mail to different servers in a priority order.
SPF (sender policy framework) record Helps prevent email spoofing and phishing.
SRV (service) record Used by Lync Online and Exchange Online to coordinate the flow of information between Office 365 services. For example, the SRV records are required to see presence in Outlook Web App, and to use Lync, Skype, or other instant messaging tools with people in other companies.
TTL (time-to-live) The amount of time that a name server keeps a DNS record before the server looks for an updated version.
Applies to:
Office 365 Enterprise admin, Office 365 Midsize Business admin, Office 365 Small Business admin, SharePoint admin center