Bdtyktl

As I understand it, a DNS server 'simply' looks up in some sort of table and find the IP-address associated with the domain you are trying to access. Then the IP-address is returned to you browser and the browser will make some http-request to that IP-address, and you get the http-response (Webpage) returned.

When I lookup or trace the IP-address of eg. wikipedia.org I get:
91.198.174.192

But when I then try to access that IP-address directly, I do not get the normal wikipedia.org frontpage, but instead I access some 'mother'-server in the Wikimedia-foundation.

So clearly some extra information must be mapped besides the raw IP-address. Can anyone explain how the server knows the difference?

There are multiple things at play here.

In the case of a website DNS does translate a domain name into an IP address so the client knows the server to communicate with. Due to the shortage if IP space, multiple websites can be hosted on a so while server so it's standards for the browser to additionally tell the server which site it wants (part of the http 1.1 spec which expands on the ancient 1.0 spec that does not do this). The specific field sent by the client to the server (one of the first) is a Host: header. Indeed if you telnet to the given IP address, then type "GET /path/to/resource.file" host: domain.name an http website will return the HTML result if the request.

DNS is, however, significantly more complex then just mapping domain names to IP addresses, including many kinds of lookips to facilitate arbirtrary text fields (which facilitate spam management, site verification and other things), mail exchange priorities, server/port assignments for services that support it, redirection and delegation.

Your browser includes in the HTTP request the domain and document it wants from the server. For example, www.wikipedia.com/index.html. The server can be running more than one website, and it will need this information to know what website and what page you want to view. If you put in 'http://91.198.174.192/', no domain nor page is sent with your request. If the server has no default domain, then you will see an error. In this case, the default domain is a small error website.

There are two instances of mapping involved in the example in your question:

• Mapping in the DNS


• Virtual host mapping

On the DNS side, the canonical DNS hostname <-> IP address mapping is only one of the possibilities, it is called the "A-Record" (or AAA for IPv6), but there exist other record types, the most prominent of which are the MX record, which holds the information which mail server is to be contacted if you want to send E-Mail to an address in a domain. There are ofcourse many others, e.g. SOA (Start of authority) and NS (Nameserver) records, which hold together the tree-structure of domains in the distributed database that is the DNS system.

The virtual host mapping is a mechanism totally independent of DNS, that allows (since HTTP version 1.1) to host different WWW-domains on the same IP address. It works by your browser accessing the IP address as fetched from DNS, but additionally sending a Host: header with the requested domain. This is the reason why you got an unexpected answer when calling the IP address directly in your browser: The address was the same, but the Host: header was different.