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Why is “ localhost” needed in HOSTS file ?

Is localhost host resolved to

If "yes", then why does everything (IIS , ASP.NET, MS SQL Server, webapps, sharepoint, etc.) work on my WIndows XP Pro SP3 exactly the same if I use (or any loopback IP 127.0..0.*) instead of

Is localhost well-known Windows identifier?

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migrated from Aug 21 '10 at 13:43

This question came from our site for professional and enthusiast programmers.

marked as duplicate by Sathya, Tom Wijsman, BinaryMisfit Aug 21 '10 at 17:20

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Added Update1 and Related questions – Gennady Vanin Геннадий Ванин Aug 21 '10 at 5:15
@vgv8: please don't add unnecessary (un)related links to your question. Readability of the question is severly impacted – Sathya Aug 21 '10 at 14:49
@moderator, I cannot post any question in SU, MSU, SO, as well as edit my posts. Is it intentional? – Gennady Vanin Геннадий Ванин Aug 21 '10 at 17:38
Submitting question in SU, SO, MSU fails with: " Oops! Your question couldn't be submitted because: users with less than 100 reputation can only post questions every 20 minutes; try again later." though I had not submitted any question for the last 24 hours – Gennady Vanin Геннадий Ванин Aug 21 '10 at 17:40
@vgv8: Moderator swill not be updated by commenting on your question, only Sathya will and he is not a moderator as he doesn't wear a diamond after his name. You could place your problem on the meta site so that a moderator sees it or contact if it's really urgent. – Tom Wijsman Aug 21 '10 at 19:03
up vote 5 down vote accepted

The Loopback device (aka Locahost) is mapped to the whole range of to, which is why it works:

The most commonly used IP address on the loopback device is for IPv4, although any address in the range to is mapped to it.


See this article on host name resolution (specific to 2003, but should apply to XP).


  • Your DNS server is responding with localhost for any 127.* query (unlikely)
  • The entry is still cached (ipconfig /flushdns to clear it out)
  • You may have an lmhosts file that maps it

I have seen that the default networks file in windows contain loopback 127, which may be why this works.

It is also quite possible that the mapping loopback / localhost is built into the TCP/IP stack itself.

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Thanks for try but see my Update1 in main question – Gennady Vanin Геннадий Ванин Aug 21 '10 at 5:06
@vgv8 - see my update. Perhaps this will provide a better answer. – Oded Aug 21 '10 at 5:25

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