 Now, we are going to look at the organization a.k.a. the hierarchy of enums because it is important to organize it in a way that the translation from telephone number to a DNS compliant format back and forth is done at certain levels which are accessible to the users and to the carriers. So here in this module we will understand what are tiers in carrier enum. First, the concept of tiers or layers as in enum is not very much relevant with regards to the user or the public enum because the user is always sitting at the edge of the network. So the concept of tiering does not readily apply there. Tiering or layering is relevant in carrier enum. So the carrier enum maps the hierarchy of the numbering in E.164 from the PSTN PLMN to NGN all IP environment for a group of users. Since it is being done with regards to a group of users from different peering carriers, so the carrier enum has to be organized as the carriers or the ISPs are organized themselves. So the carrier enum is based on three tiers. Each tier has a database or a registry. So like we had the root node in DNS, we have tier 0 at the root. We have tier 1 and tier 2 in the hierarchy. Tier 1 is at more regional level and tier 2 is more at the country or organizational level. Tier 0 is internationally centralized root directory. All the operators, carriers, service providers query this tier 0 database to actually get information about authoritative sources of routing information. It is similar to looking for authoritative name servers from the DNS root server. All these authoritative resources are scattered around the world at different tiers, tier 1 and tier 2. Now the tier 1 consists of national registry for enum and tier 2 is actually defined as a national level database that is distributed within a certain country. So tier 1 and tier 2 are like the country level top domain and the country specified domains. Let's look at this diagram. This diagram is again from Tony Jenevsky. Here we can see that on the left hand side we have a figure that is in four quadrants. Three of these quadrants including the private enum, infrastructure enum for the user uniform resource identifiers that is the URLs and the interconnect or the carrier or ISP URIs are under the jurisdiction of the carrier enum. As far as the user enum is concerned, it is public data and it always is suffixed with a telephone number written in reverse order ending with e.164.ARPA. On the right hand side approximate understanding of how the tiers are organized at the international country and within country level are shown here. We have tier 0, tier 1 and tier 2. This pertains to carrier enum only.