Microsoft's Active Directory needs DNS in order to perform properly. If you have ever had DNS issues in an AD domain you will know what I am talking about! Here are a few I see ALL the time.
An organization has an Active Directory domain called joesdomain.com and hosts their website externally with a webhost. When an user on the LAN types in www.joesdomain.com they get a 404 error. To correct this you need to create an A record in your DNS servers forward lookup zone. Name the A record WWW and for an IP address, enter the IP address of your website. You should see a record like this in your forward lookup zone when complete.
WWW (Host)A 65.80.23.4
This can also be done if you host your email externally and need to resolve POP3.domain.com (or SMTP IMAP or any other name)
POP3 (Host)A 65.80.23.5
Another common issue is slooooowwwwwwww logins to an Active Directory domain. The number one culprit here? Misconfigured DNS on the client PC. If the TCP/IP settings for a client computer specify the IP address of a public DNS server then the TCP/IP resolver won't be able to find Service Locator (SRV) records. These records advertise the domain controller services, LDAP, Kerberos and Global Catalog. Without these records, the client computer cant authenticate and get the information it needs to operate in the domain.
Active Directory domain clients should ALWAYS have their primary and secondary DNS IP's set to the internal DNS servers. These DNS servers can be setup with Root Hints and/or forwarders to an external DNS server(s) A common cause of this is admins who use a broadband router (Linksys, D-Link etc...) to handle DHCP for their LAN. There is nothing wrong with these devices, however they do not hand out the internal DNS servers IP addresses. Some are configurable to do so, but most just hand out the ISP's DNS server IP(s). If you don't have the resources to install DHCP on one of your servers, you can set the primary and secondary DNS IP's on the clients statically. The static address will override the one issued via DHCP.