We talked about DNS problems at Fallure to resolve, but there are other reasons why a
particular machine may be unable to browse the Internet while others
on the same lan still can.
First, the same DNS advice applies as in the above article: check
web access with a numeric IP address. If that works, you just lack proper DNS.
With Windows XP, a user with access to networking properties can
easily override default settings from DHCP while retaining their
correct machine ip address, so it is worth checking.
Just one silly thing because I've seen this more than once: just because
your browser's default page doesn't come up doesn't mean nothing works. I've had customers insist that that their internet was down when really it was just
their default home page! Check some other site first.
You should also check that a misconfigured firewall on the
machine itself isn't blocking outgoing connection attempts or incoming
data. If in doubt, temporarily shut off the local firewall to
test. Test wth telnet from the command line: "telnet xyz.com 80"
If Windows, is it really that you can't access the Internet or
only that Internet Explorer cannot? Microsoft's IE is a fragile
and rather brain-dead thing, easily confused and easily broken.
Under Internet Options, Connections, Lan Settings, see if
Detect Settings Automatically is set. If it is, IE is going to
look for a proxy server before it pays attention to its default route.
If you have a proxy server, and intend to use it, that's fine, but
if more than one proxy exists, IE may pick up the wrong one. Try
If IE still isn't working, try downloading Firefox or any other
browser. If the other browser works, it's all IE's fault. Is
anyone really surprised?
Some lan firewalls have access lists. It's not uncommon to
configure a firewall not to grant access to any machine it did not
provide a DHCP address for. If the user over-rode their DHCP address,
or if an accidental DHCP server appeared on the network, the user
may be blocked (or may just have misconfigured themselves).
The right default route is of course important. I've seen
this get screwed up when a lan was split for security or
performance reasons: what was formally all 192.168.1.x is now
192.168.1.x and 192.168.2.x. The new lan was added to the DHCP server,
but they used the same default gateway everywhere. That won't
work: the gateway for 192.168.2.x machines needs to be in the 192.168.2.0
An old or forced arp entry at the firewall that incorrectly
identifies a pc will prevent that machine from passing through the
router. Power cycling the router is a quick fix if there is any
chance of that being true.
If in a corporate environment, you may be blocked at a smart
switch. Try switching a working machine to that port, keeping
in mind that the port could be allocated to a specific mac address.
Anything else? Sure - all kinds of strange permissions and
corruption issues that could affect a machine. Viruses and other
malware can do strange things with network access. These can
be tough to spot and hard to fix reliably.
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