# Understanding Authoritative Name Servers and DNS Zones

We've covered how authoritative name servers are responsible for responding to name resolution requests for specific domains, but they do more than that. An authoritative name server is actually responsible for a specific DNS zone.

## Hierarchical Structure of DNS Zones

DNS zones are a hierarchical concept. The root name servers we covered earlier are responsible for the root zone. Each TLD name server is responsible for the zone covering its specific TLD. What we referred to as authoritative name servers are responsible for even finer grained zones underneath that. The root and TLD name servers are actually just authoritative name servers too, but the zones they have authority for are special cases.

## Non-overlapping Zones

It's essential to note that zones don't overlap. For example, the administrative authority of the TLD name server for the .com TLD doesn't encompass the google.com domain. Instead, it ends at the authoritative server responsible for google.com.

## Purpose of DNS Zones

The purpose of DNS zones is to allow for easier control over multiple levels of a domain. As the number of resource records in a single domain increases, it becomes more of a headache to manage them all. Network administrators can ease this pain by splitting up their configurations into multiple zones.

## Example: Large Company Scenario

Let's imagine a large company that owns the domain largecompany.com. This company has offices in Los Angeles, Paris, and Shanghai. To manage the vast number of resource records efficiently, the company can split each office into its own zone. Now, we can have la.largecompany.com, pa.largecompany.com, and sh.largecompany.com as subdomains, each with its own DNS zone. This setup would require a total of four authoritative name servers: one for largecompany.com and one for each of the subdomains.

## Configuration via Zone Files

Zones are configured through what are known as zone files, simple configuration files that declare all resource records for a particular zone. A zone file must contain an SOA (Start of Authority) resource record declaration, which declares the zone and the name of the name server that is authoritative for it. Additionally, NS (Name Server) records indicate other name servers that might also be responsible for this zone.

## Redundancy and Failover

While we've been referring to servers in the singular when discussing what's responsible for a zone, there are often multiple physical servers with their own Fully Qualified Domain Names (FQDNs) and IP addresses involved. Redundancy is crucial in DNS setups. If one server were to have a problem or suffer a hardware failure, other servers can serve DNS traffic.

## Record Types and Configurations

In addition to SOA and NS records, zone files contain other resource record types such as A (IPv4 address), AAAA (IPv6 address), and CNAME (canonical name) records. These files also include configurations such as default TTL (Time to Live) values for the records served by this zone.

## Depth of Zones

Just like subdomains can go many layers deep, zones can also be configured similarly. However, it's rare to see zones deeper than just a few levels.

## Reverse Lookup Zones

Sometimes, you'll encounter what are known as reverse lookup zone files. These allow DNS resolvers to ask for an IP and get the Fully Qualified Domain Name (FQDN) associated with it returned. Reverse lookup zone files are similar to regular zone files but primarily contain PTR (Pointer) resource record declarations, which resolve an IP to a name.
