... connect to the www.mydomain.com Web server. This is all made more efficient by the fact that the DNS servers cache the returns they get to their queries, so that if the same name is queried again, the client's web server doesn't have to go through the whole process because it already has the results of the query for the IP address of www.mydomain.com in its cache.
NOTE
Information doesn't stay in the cache indefinitely. After a specified time, called the Time To Live or TTL, the cached information is removed.
Configuring DNS for scalability
DNS information can be replicated across multiple DNS servers using zone transfers. For fault tolerance, your network should have at least two DNS servers configured for each DNS zone (usually a DNS domain or subdomains). One is the primary DSN server, where the master zone file is stored. The other is the secondary DSN server, and it gets its DNS database information from the master, or authoritative DNS server for that zone. You can also have caching-only DNS servers that don't have their own database files but only cache DNS information or forwarders that forward name queries to other DNS servers.
When you set up a DNS infrastructure for your internal network, one big consideration is the namespace. If the DNS will be used only for internal queries, you don't have to register the domain name (but there are advantages to doing so — especially if you plan to someday use the domain name for servers that will available on the Internet).
When your business grows to the point where you want to provide services to users over the Internet as well as those on the internal LAN, you have several choices:
- Use different domain names for your internal and external namespaces Use an internal domain that is a subdomain of the external namespace Use the same domain name for internal and external namespaces, and configure a split DNS infrastructure. With split DNS, you create two different zones for the same domain.
You can find an explanation of how split DNS works here.
For scalability, you should plan your DNS namespace in a logical hierarchy so that you can easily add new zones without adversely affecting the extant namespace. It's best not to extend the namespace more than five levels below the root (Internet or internal) domain.
If your internal network crosses multiple physical locations, the best option may be to use an internal DNS root zone that is authoritative for all the top-level domains on the network and is isolated from the Internet. The DNS namespace can be hosted on multiple DNS servers for scalability. In an enterprise environment, you can create a DNS infrastructure with multiple top level domains.
For more information about designing a DNS infrastructure for the enterprise with Windows DNS servers, see the Microsoft TechNet web site here.
