Dear experts,
when i run apt-get update in terminal,
following errors displayed



Err http://archive.canonical.com natty InRelease

Err http://extras.ubuntu.com natty InRelease

Err http://extras.ubuntu.com natty Release.gpg
Unable to connect to extras.ubuntu.com:http:
Err http://archive.canonical.com natty Release.gpg
Unable to connect to archive.canonical.com:http: [IP: 91.189.92.191 80]
Err http://ppa.launchpad.net natty InRelease

Err http://ppa.launchpad.net natty Release.gpg
Unable to connect to ppa.launchpad.net:http:
Err http://archive.ubuntu.com natty InRelease

Err http://archive.ubuntu.com natty-updates InRelease

Err http://archive.ubuntu.com natty-security InRelease

Err http://archive.ubuntu.com natty Release.gpg
Unable to connect to archive.ubuntu.com:http: [IP: 91.189.92.181 80]
Err http://archive.ubuntu.com natty-updates Release.gpg
Unable to connect to archive.ubuntu.com:http: [IP: 91.189.92.181 80]
Err http://archive.ubuntu.com natty-security Release.gpg
Unable to connect to archive.ubuntu.com:http: [IP: 91.189.92.181 80]
Reading package lists... Done
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/natty/InRelease

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dis...ates/InRelease

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dis...rity/InRelease

W: Failed to fetch http://archive.canonical.com/ubuntu/...atty/InRelease

W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/natty/InRelease

W: Failed to fetch http://ppa.launchpad.net/mozillateam...atty/InRelease

W: Failed to fetch http://archive.canonical.com/ubuntu/...ty/Release.gpg Unable to connect to archive.canonical.com:http: [IP: 91.189.92.191 80]

W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/natty/Release.gpg Unable to connect to extras.ubuntu.com:http:

W: Failed to fetch http://ppa.launchpad.net/mozillateam...ty/Release.gpg Unable to connect to ppa.launchpad.net:http:

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dis...ty/Release.gpg Unable to connect to archive.ubuntu.com:http: [IP: 91.189.92.181 80]

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dis...es/Release.gpg Unable to connect to archive.ubuntu.com:http: [IP: 91.189.92.181 80]

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dis...ty/Release.gpg Unable to connect to archive.ubuntu.com:http: [IP: 91.189.92.181 80]

W: Some index files failed to download. They have been ignored, or old ones used instead.

root@summit-Inspiron-N5010:/var/lib/apt# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 11.04
Release: 11.04
Codename: natty


Can anyone suggest me what I do to sort out this?
Summit

Try
sudo aptitude update or sudo aptitude safe upgrade?

Is the machine connected to the internet? What happens when you try to open the links such as:
http://archive.ubuntu.com/ubuntu/dis...ty/Release.gpg
If the machine is correctly connected to the internet that should open a PGP signature on ubuntu.com.

again same error

yes, internet is connected. When I click the link
http://archive.ubuntu.com/ubuntu/dis...ty/Release.gpg

following message displays:

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)

iEYEABECAAYFAk22t8MACgkQQJdur0N9BbWDzgCghFVNKkh8wskACvTceMMkzM7+
DMAAoJOzHHdCt8escDyqII5EgdVkBNJC
=BEdw
-----END PGP SIGNATURE-----

Do you use proxy? Or direct connection ?
But if I were you, the first thing I started from - was to select another mirror. The easiest way to do this is to choose another one from the update-manager settings.

Well, the message is correct in that the following does not exist:
http://archive.ubuntu.com/ubuntu/dists/natty/InRelease
I'll admit I'm not up on how Ubuntu is right now but you should compare your sources.list with a known good one to ensure the entries are correct. I think some of the PPAs may have stopped supporting Ubuntu 11.04 (Natty) or support it through another URL.
Hopefully someone with more Ubuntu knowledge will be able to point out the sources,list changes needed.

Try changing the download server. And re-execute the command. Click on System--> Administration-> Synaptic Package Manager-> Settings-> Repositories > Download from. Change it from what you have to Main Server or select Other > Choose Best server.

It should be noted that Ubuntu 11.04 is getting to be a bit long and the tooth and I am not sure if it's supported any more. It might not be a bad idea to try to upgrade to 12.04.