Web Hosting Glossary - DNS Print

Introduction to DNS

The Domain Name System, or DNS, is the means by which computers connected to the Internet get information about each other. The individual pieces of information are known as records; each record is of a certain type. Computers look up records for a domain by asking the name server for the domain about the records relevant to that domain.

IP addresses are the numbers which identify computers to each other.

Time Till Stable

This is an indication of the amount of time until you can be really sure that the change will be visible via all ISP DNS caches. The amount of time that our DNS servers tell remote DNS caches to wait for is much shorter than this, however if you've just transferred the domain name in, several DNS caches will still be using the timeout that the old host's DNS servers suggested, which is conventionally no longer than 24 hours.

If the TTS value is zero, that means that all remote caches should be up to date, so if you're still seeing the old value that means there's probably a problem (eg. you entered invalid DNS records). If the TTS value is above zero and you're still seeing the old value, it's more likely that your ISP's DNS cache just hasn't updated yet.

DNS Record Types

A records

These contain a mapping from a name to an IP address. An A record does not in itself mean that any particular service is available from the computer at that address; it just translates the name to the IP address.

CNAME records

These contain a mapping from one name (known as an alias) to another name (known as a CNAME, or canonical name). When a computer looks up records for the alias, it is given the records for the cname instead.

For example, if we set up a CNAME record for "managethisdomain.com" with alias "web" and cname "www", then all queries for "web.managethisdomain.com" would be given the information for "www.managethisdomain.com". A cname can be a name within the same domain, as in our example, or it can be a full name, like "www.google.com."; the dot on the end shows that this is a full name.

MX records

These say where email for a domain is to be delivered. A domain can have several MX records; each one has a priority from 0 to 100. Email is delivered to the one with the lowest number first, and to any others only if the first one cannot accept it. For example, there is an MX record for "managethisdomain.com" pointing to "mail.managethisdomain.com", with priority 10. This causes our email to be delivered to "mail.managethisdomain.com".

TXT records

These give miscellaneous textual information about a domain; the most common use of them is for Sender Policy Framework (SPF), which enables you to specify which computers are allowed to send email which claims to be from your domain. For more information about SPF see the SPF Project.

SRV records

These records allow applications to locate services by giving the address and port information required. They also allow the load to be shared among several different servers using the priority and weight values.

AAAA records

These are roughly equivalent to A records which express the address as an IPv6 address (hence 4 times the size of an IPv4 address). The allowed formats for IPv6 addresses follow these rules:

Standard Form: The format is x:x:x:x:x:x:x:x. This form consists of eight hexadecimal numbers, each of which expresses a 16-bit value (i.e., each x can be up to 4 hex digits).
Abbreviated Form: A series of contiguous zero values in the Standard Form can be abbreviated to ::. Only one instance of :: can occur in an address. For example, the loopback address 0:0:0:0:0:0:0:1 can be abbreviated as ::1. The wildcard address, consisting of all zeroes, can be written as ::.
We do not currently support the IPv4 mapped form (x:x:x:x:x:x:d.d.d.d).

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