DNS Namespace Hierarchy
DNS Namespace Hierarchy Like the UNIX file system, DNS domains are organized as a set of descending branches similar to the roots of a tree. Each branch is a domain, each subbranch is a subdomain. The terms domainand subdomainare relative. Apr 26, · Alternatively referred to as a namespace, a domain namespace is a name service provided by the Internet for Transmission Control Protocol networks/Internet Protocol (TCP/IP). DNS is broken up into domains, a logical organization of computers that exist in a larger network. Below is an example of the hierarchy of domain naming on the Internet.
The entire collection of DNS administrative domains throughout the world are organized in a hierarchy called the DNS namespace. This section shows how the namespace organization affects both local domains and the Internet. Each branch is a domain, each subbranch is whzt subdomain. The terms domain and subdomain are relative.
A given domain is a subdomain relative to those domains above it in the hierarchy, and a parent domain to the subdomains below it.
Or you could just as easily say that those are subdomains relative to the com domain. A domain contains one parent or top domain plus the associated subdomains if any. Domains are named up the tree starting with the lowest deepest subdomain and ending with the root domain. For example, Mktg. If your ahat is large enough, it may support a number of domains,organized into a local namespace. Figure shows a domain hierarchy that might be in place nsmespace a single company.
The top-level, or "root" domain for the organization is ajax. DNS clients request service only from the servers that support their domain. If the domain's server does not have the information the client needs, it forwards the request to its parent server, which is the server in the next-higher domain in the what is the best way to email photos. If the request reaches the top-level server, the top-level server determines whether the domain is valid.
If it is not valid, the server returns a "not found" type message to the client. If the domain is valid, the server routes the request down to the server that supports that domain. The domain hierarchy shown in Figure is, conceptually, a "leaf" of the huge DNS namespace supported on the global Internet. It consists of the root directory, represented as a dot. Note that the com domain introduced in Figure is one of a number of top-level organizational domains in existence on the Internet.
At the present time, the organizational hierarchy divides its namespace into the top-level domains listed shown in Table It is probable that additional top-level organizational domains will be added in the future. The geographic hierarchy assigns each country in the world a two- or three-letter identifier and provides official names qhat the geographic regions within each country. For example, domains in Britain are subdomains of the uk top-level domain, Japanese domains are subdomains of jpand so on.
The Internet root domain, top-level domains organizational and geographical are maintained by the various Internet governing bodies. People with networks of any size can "join" the Internet by registering their domain name in either the organizational or the geographical hierarchy. Every DNS domain must dna a domain name. If your site namfspace to use DNS for name service without connecting to the Internet, you can use any name your organization wants for its your domains and subdomains, if applicable.
However, if your site plans wants to join the Internet, it must register its domain name with the Internet governing bodies. Register your DNS domain name with the an appropriate Internet governing body. You can communicate directly with the appropriate Internet governing body or their agent. ISPs provide a wide range of services from consulting to actually hosting your Internet presence.
Domain names indicate a domain's position in the overall DNS namespace, much as path names indicate a file's position in the UNIX file system. After your local domain is registered, its what does the zero water tester test for is prepended to the x of the Internet hierarchy to which it belongs.
For example, the ajax domain shown in Figure has been registered as part of the Internet com hierarchy. Therefore, its Internet domain name becomes ajax. Figure shows the position of the ajax. DNS does not require domain names to be capitalized, though they may be. Here are some examples of nameapace and domain names:. The Internet organization regulates administration of its domains by granting each domain authority over the names of its hosts and by expecting each domain to delegate authority to the levels below it.
Thus, the com domain has authority over the names of the hosts in its domain. It also authorizes the formation of the Ajax. The Ajax. A domain name is said to be fully-qualified when it includes the names of every DNS domain from the local domain on up to ".
Conceptually, the fully-qualified domain name indicates the path to the root, as does the absolute path name of a UNIX file. However, fully-qualified domain names are read from lowest, on the left, what are ordinal numbers for kids highest, on the right.
Wyat, a fully-qualified domain name has the syntax:. DNS Hierarchy in a Local Domain If your aa is large enough, it may support a number of domains,organized into a local namespace.
Figure Hierarchy of Internet Domains At the present time, the organizational hierarchy divides its namespace into the top-level domains listed shown in Table Table Internet Organizational Domains Domain Purpose com Commercial organizations edu Educational institutions gov Government institutions mil Military groups net Major network support centers org Nonprofit organizations and others int International organizations The geographic hierarchy assigns each country in the world a two- or three-letter identifier and provides official names for the geographic regions within each country.
Joining the Internet The Internet root domain, top-level domains organizational and geographical are maintained by the various Internet governing bodies. Obtain a network IP address from that governing body. There are two ways to accomplish this: You can communicate directly with the appropriate Internet governing body or their agent. Here are some examples of machines and domain names: Boss.
Fully-Qualified Domain Names A domain name is said to be fully-qualified when it includes the names of every DNS domain from the local domain on up to ". Therefore, a fully-qualified domain name has the syntax: The fully qualified domain names for the ajax domain and its subdomains are: ajax.
The resolution of names through the use of Domain Name System (DNS) is central to Windows operation. Without proper name resolution, users can't locate resources on the network. The DNS hierarchical inverted tree structure is called the DNS namespace. The DNS namespace tree has a unique root and large number of sub-trees. A domain is a subtree of the DNS name space. A Domain can further divided in to sub-domains. A DFS namespace is a virtual shared folder that contains shared folders from multiple servers. This allows users to access shared folders using the Universal Naming Convention (UNC) path \\ [Domain]\ [Namespace] instead of remembering the name of each server to which to connect.
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I've read the man page, but I do not understand what name and namespace are for. For version 3 and version 5 UUIDs the additional command line arguments namespace and name have to be given. The name is a string of arbitrary length. In either case, why is this not done automatically? What is the purpose of it then? Can it be decoded from the UUID v5? Roughly speaking, a type 3 or type 5 UUID is generated by hashing together a namespace identifier with a name.
Only bits are available and 5 bits are used to specify the type, so all of the hash bits don't make it into the UUID. Also MD5 is considered cryptographically broken, and SHA1 is on its last legs, so don't use this to verify data that needs to be "very secure".
Suppose you have a key,value store, but it only supports one namespace. You can generate a large number of distinct logical namespaces using type 3 or type 5 UUIDs.
First, create a root UUID for each namespace. These UUIDs can be thrown into a single key-value store with high probability of avoiding collision. This process can be repeated recursively so that if for instance the "value" associated with a UUID key in turn represents some sort of logical "namespace" like a bucket, container or directory, then its UUID can be used in turn to generate more hierarchical UUIDs.
The generated type 3 or type 5 UUID holds a partial hash of the namespace id and name-within-namespace key. The name is an "arbitrary" octet string from the perspective of the uuid algorithm.
Its meaning however depends on your application. It could be a filename within a logical directory, object-id within an object-store, etcetera. While this works well for a moderately large number of namespaces and keys, it eventually runs out of steam if you are aiming for a very large numbers of keys that are unique with very high probability.
The Wikipedia entry for the Birthday Problem aka Birthday Paradox includes a table that gives the probabilities of at least one collision for various numbers of keys and table sizes. Adjusting for 5 bits that encode the UUID type, it will run out somewhat faster, so a trillion keys have roughly a 1-in-a-trillion chance of having a single collision.
You're probably wondering what is it that I'm supposed to hash. Basically you hash the concatenation of:. Now that you have a function that generates a so-called Name , you can have the function in pseudo-code :.
The namespace is whatever UUID you like. It can be one of the pre-defined ones, or you can make up your own, e. The name is just the text you want to have appended to the namespace, then hashed, and stuffed into a UUID:. A name is nothing more than an identifier that is unique within some namespace. The problem is that namespaces are often quite small and the names in one often collide with names in others. For instance, my car's license plate number name is unique within my state DMV's namespace, but it's probably not unique in the world; other state DMVs may have used the same name in their own namespaces.
Heck, someone else may have a phone number name that also matches because that's yet another namespace, etc. UUIDs can be seen as inhabiting a single namespace so vast that it can provide a unique name for everything ; that's what the "universal" means. But how do you map existing names in other namespaces to a UUID? One obvious solution is to generate a UUID V1 or V4 for every item to replace the old names in their disjoint namespaces.
The downside is that they're a lot bigger, you have to communicate all the new names to everyone who has a copy of your dataset, update all your APIs, etc. Odds are you can't actually get rid of the old names entirely anyway, which means now every item has two names, so did you make things better or worse?
You don't need to publish them at all nor even pre-generate them since anyone can create them on the fly as needed! For all other namespaces, you have to generate your own namespace UUID V1 or V4 and communicate it to anyone who needs it. If you have several namespaces, having to publish the UUID for each one is clearly not ideal. This is where the hierarchy comes in: you create one "base" UUID of whatever type , and then use that as a namespace for naming your other namespaces!
That way, you only have to publish the base UUID or use an obvious one , and everyone can calculate the rest. StackOverflow itself has separate namespaces for users, questions, answers, comments, etc.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Generating v5 UUID. What is name and namespace? Ask Question. Asked 8 years, 10 months ago.
Active 12 days ago. Viewed 87k times. Nikita Fedyashev Gajus Gajus Add a comment. Active Oldest Votes. Name and namespace can be used to create a hierarchy of very probably unique UUIDs. BuZZ-dEE 3, 6 6 gold badges 47 47 silver badges 76 76 bronze badges. Any performance issues I should know about? I am new to the concept and was puzzled of what that hierarchy you're talking about is. Where can I see it etc Some clarity came once I stuck on explanation this might be used to create a reproducible UUID for namespace.
Ian Boyd Ian Boyd k gold badges silver badges bronze badges. Thanks for the thorough explanation. Is it possible to decode the name or the namespace decoded from the UUID? Sathesh No, it is not possible to decode a hash; hashes are one-way functions.
There is no way to decode that byte hash back into 81 GB. If you really needed to, you can try hashing all possible GUIDs and possible strings until you find the combination that gives the same result. With any luch you'll find it somewhere between forever and eternity. Eugen Konkov StephenS StephenS 7 7 silver badges 11 11 bronze badges.
I am wondering why stackoverflow needs to map a uniquely generated big integer to a UUID given that its APIs apparently return only the big integer as a string.
Why do this hybrid strategy. This is a fantastic answer. Thank you. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. Podcast Non-fungible Talking. Featured on Meta. New onboarding for review queues. Outdated Answers: results from use-case survey. Downvotes Survey results. Linked See more linked questions.