Well, I have never asked or been asked it, but these are the steps I would have taken. Also, I use windows, so there might be other stuff that would work better than these..
Any of the 1-7 questions should locate what the major issue could be, and could be asked in whatever order is best suited
- Can I access any other webpage?
- If yes: that particular site might be down
- If no: something else is wrong
- Is anyone else affected by this issue? (ask collegues if they can access the internet)
- If yes: might be a corporate thing [End questions: I would take a coffee and wait until it works for someone else]
- If no: this issue could be local for my computer
- Check Is it down for me or everyone? (goes hand in hand with the first question.. and the second one)
- If down for everyone: not much you can do unless you administer that site
- If down for me: something else is wrong
- Ping the DNS
- If the DNS responds: something else is wrong
- If it doesn't respond you'll won't be able to ping (for instance) www.yahoo.com, but maybe the actual ipadress
- Find the ipadress to the site that is affected and try to ping that one
- If it responds: [End questions: the DNS is the issue]
- If it does not respond: something else is wrong
- Ping something inside your network for instance a colleagues ipaddress
- If it does not respond: your network is the issue.
- If it does respond: you at least have local network
- Ping something outside your network Google is usually up. (goes hand in hand with question 4)
- If it responds: probably the site that have issues
- If it does not respond: Something else is wrong
- Has it worked before? Actually a valid question.
- If it has not worked before: could be network settings for local computer
- If it has worked before: what settings have changed since last time?
so now you should know if the site is not working, if internet is not working, if the DNS is functional and if you at least have intranet.
Note that Is it down for me or everyone? tests port 80, so it might just be the web site that is down. This will not work if you for instance use any other port for the web interface or if it is a terminal server or whatever.
Is it just the site?
As mentioned before, grab a coffee
Is internet down but not local network?
Some networks won't let computers out on the internet if they do not match with ipadresses-macaddresses.
You could have a static ipadress instead of a dynamic one (if the DNS is wrong, this would also be a valid reason for that, since when you set a static ipaddress, you also need to set the dns)
Restart the computer.
Compare settings with the colleague who's computer responds.
AS you say, firewall.
And a bonus: Check your network for a local router (with DHCP enabled). Seriously, I've worked in support for 2 years and it has actually happened more than once... This messes up some networks really bad..
Is internet and local network down?
I guess it depends if anyone else has the problem as well, but if it's only you:
The network ipsettings would also come in handy here.
Check the event log for any suspicious network logs.
Don't know about unix, but I usually restart the computer.. grab a coffee..
And if nothing else works, sweet talk it, tell it that it is your favorite computer and that you love it very much, that you'll stop surfing on the CPU demanding sites and it should let you out again ;)