Cisco Network Registrar User's Guide, 6.2.1
13 - Introduction to DNS
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Introduction to the Domain Name System

Table Of Contents

Introduction to the Domain Name System

How DNS Works


Learning ExampleCo's Address

Establishing a Domain

Difference Between Domains and Zones


Reverse Nameservers

High Availability DNS

Introduction to the Domain Name System

The Domain Name System (DNS) was designed to handle the growing number of Internet users. DNS translates names, such as, into IP addresses, such as (or the more extended IPv6 addresses), so that computers can communicate with each other. DNS makes using Internet applications, such as the World Wide Web, easy. The process is as if, when phoning your friends and relatives, you could autodial them based on their names instead of having to remember their phone numbers.

How DNS Works

To understand how DNS works, imagine a typical user, John, logging on to his computer. He launches his Web browser so that he can view the website at a company, ExampleCo (see Figure 13-1). He enters the name of their website— Then:

1. John's workstation sends a request to the DNS server about the IP address of

2. The DNS server checks its database to find that corresponds to

3. The server returns this address to John's browser.

4. The browser uses the address to locate the website.

5. The browser displays the website on John's monitor.

Figure 13-1 Domain Names and Addresses


John can access ExampleCo's website because his DNS server knows the IP address. The server learned the address by searching through the domain namespace. DNS was designed as a tree structure, where each named domain is a node in the tree. The top-most node of the tree is the DNS root domain (.), under which there are subdomains, such as .com, .edu, .gov, and .mil (see Figure 13-2).

Figure 13-2 The Domain Name System Hierarchy

The fully qualified domain name (FQDN) is a dot-separated string of all the network domains leading back to the root. This name is unique for each host on the Internet. The FQDN for the sample domain is, with its domain example, parent domain .com, and root domain "." (dot).

Learning ExampleCo's Address

When John's workstation requests the IP address of the website (see Figure 13-3):

Figure 13-3 DNS Hierarchical Name Search

1. The local DNS server looks for the domain in its database, but cannot find it, indicating that the server is not authoritative for this domain.

2. The server asks the authoritative root nameserver for the top-level (root) domain "." (dot).

3. The root nameserver directs the query to a nameserver for the .com domain that knows about its subdomains.

4. The .com nameserver determines that is one of its subdomains and responds with its server's address.

5. The local server asks the nameserver for's location.

6. The nameserver replies that its address is

7. The local server sends this address to John's Web browser.

Establishing a Domain

ExampleCo has a website that John could reach because it registered its domain with an accredited domain registry. ExampleCo also entered its domain name in the .com server's database, and requested a network number, which defines a range of IP addresses. In this case, the network number is, which includes all addresses in the range through You can only have the numbers 0 through 256 (28) in each of the address fields, known as octets. However, the numbers 0 and 256 are reserved for network and broadcast addresses, respectively, and are not used for hosts.

Difference Between Domains and Zones

The domain namespace is divided into areas called zones that are points of delegation in the DNS tree. A zone contains all domains from a certain point downward, except those for which other zones are authoritative.

A zone usually has an authoritative nameserver, often more than one. In an organization, you can have many nameservers, but Internet clients can query only those that the root nameservers know. The other nameservers answer internal queries only.

The ExampleCo company registered its domain, It established three zones—,, and ExampleCo delegated authority for and to the DNS servers in the Marketing and Finance groups in the company. If someone queries about hosts in, directs the query to the nameserver.

In Figure 13-4, the domain includes three zones, with the zone being authoritative only for itself.

Figure 13-4 With Delegated Subdomains

ExampleCo could choose not to delegate authority to its subdomains. In that situation, the domain is a zone that is authoritative for the subdomains for marketing and finance (see Figure 13-5). The server answers all outside queries about marketing and finance.

Figure 13-5 Without Delegation

As you begin to configure zones by using Network Registrar, you must configure a nameserver for each zone. Each zone has one primary server, which loads the zone's contents from a local configuration database. Each zone can also have any number of secondary servers, which load the zone contents by fetching the data from the primary server. Figure 13-6 shows a configuration with one secondary server.

Figure 13-6 Primary and Secondary Servers for Zones


DNS is based on a client/server model. In this model, nameservers store data about a portion of the DNS database and provide it to clients that query the nameserver across the network. Nameservers are programs that run on a physical host and store zone data. As administrator for a domain, you set up a nameserver with the database of all the resource records (RRs) describing the hosts in your zone or zones (see Figure 13-7). For details about DNS RRs, see "Resource Records."

Figure 13-7 Client/Server Name Resolution

The DNS servers provide name-to-address translation, or name resolution. They interpret the information in a fully qualified domain name (FQDN) to find its address. If a local nameserver does not have the data requested in a query, it asks other nameservers until it finds it. For commonly requested names, this process can go quickly, because nameservers continuously cache the information they learn from queries about the domain namespace.

Each zone must have one primary nameserver that loads the zone contents from a local database, and a number of secondary servers, which load a copy of the data from the primary server (see Figure 13-8). This process of updating the secondary server from the primary server is called a zone transfer.

Figure 13-8 DNS Zone Transfer

Even though a secondary nameserver acts as a kind of backup to a primary server, both types of servers can be authoritative for the zone. They both learn about hostnames in the zone from the zone's authoritative database, not from information learned while answering queries. Clients can query both servers for name resolution.

As you configure Network Registrar's DNS nameserver, you specify what role you want the server to perform for a zone—primary, secondary, or caching-only. The type of server is meaningful only in context to its role. A server can be a primary for some zones and a secondary for others. It can be a primary or secondary only, or it can serve no zones and just answer queries by means of its cache.

Although all servers are caching servers, because they save the information until it expires, a caching-only server is one that is not authoritative for any zone. This server answers internal queries and asks other authoritative servers for the information. Sites create caching-only servers to unburden the authoritative servers so that they do not need to have every query directed to the authoritative servers.

To configure the:

Primary nameserver, see the "Managing Primary DNS Servers" section.

Secondary server, see the "Managing Secondary Servers" section.

Caching-only server, see the "Configuring Caching-Only Servers" section.

Reverse Nameservers

The DNS servers described so far perform name-to-address resolution. They can do this easily by searching through their database for the correct address, because they index all the data by name. However, there are times when you need address-to-name resolution so that you can interpret certain output, such as computer log files.

Finding a domain name when you only know the address, however, would require searching the entire namespace. DNS solves this problem by supporting a domain namespace that uses addresses as names, known as the domain. This reverse zone contains subdomains for each network based on the network number. For consistency and natural grouping, the four octets of a host number are reversed.

The IP address as a domain name appears backward, because the name is in leaf-to-root order. For example, ExampleCo's example domain's network number is Its reverse zone is If you only know the DNS server address (, the query to the reverse domain would find the host entry that maps back to

Reverse domains are handled through Pointer (PTR) RRs, as indicated in Figure 13-9.

Figure 13-9 Reverse Domains

High Availability DNS

Because there can be only one primary DNS server per zone, you risk the failure of dynamic updates if the primary DNS server goes down. These updates can occur on the primary DNS server only; a secondary DNS server cannot record these changes, but must forward them to the primary. To solve this problem, a second primary server can become a hot standby that shadows the main primary. This is called High Availability (HA) DNS (see "Configuring High Availability DNS Servers"). Both servers in this failover configuration must synchronize so that their primary zones and related attributes are identical. Network Registrar provides settings on the main server to identify the main and backup for synchronization, and the time-out period to go over into failover mode.