Generic Top Level Domain (gTLD)

The generic top level domains (gTLD) specify types of services. The most widely used gTLD is .com, which generally designates a commercial-oriented entity. Other common gTLDs include .net and .edu for network and educational services, respectively. Originally there were only six gTLDs [RFC1032], but more have since been added.

A few gTLDs are treated as reserved for testing, but they are not officially reserved domains. These include .example, .invalid, .localhost, .localdomain, and .test. The .lan gTLD is commonly used for private, internal networks. Any gTLD that is not known by the root servers operates privately.

As with the root servers, there are many gTLD servers. These have letters for hostnames and exist in the gtld or gtld-servers domain of their respective gTLDs, and not every gTLD is recognized by all root servers. For example, the .pro gTLD is managed by a.gtld.pro, b.gtld.pro, and c.gtld.pro. Only the A, G, and J root servers identify this gTLD. A simple script (Listing below) can be used to identify the list of gTLD servers.

Shell Script to List gTLD Servers

#!/bin/sh
# for each root server
for root in a b c d e f g h i j k l m ; do
  echo "$root.root-servers.net"
  # for each gTLD
  for gtld in com edu gov net org mil arpa \
    int aero biz coop info museum name pro ; do
    # List the gTLDs know by the root server
    host -t ns $gtld $root.root-servers.net | \
      grep -v ":" | grep server
  done
done