In order to connect to the internet you need to have several things going for you.
You need to have a working network connection from your computer to your gateway.
Your gateway needs to be working properly.
You need to have proper IP addresses and to be able to connect effectively using TCP/IP.
You need to have DNS server addresses configured correctly.
Your DNS service needs to be working.
You need to be able to contact your DNS service.
So if any of those things are not working correctly then you won't be able to surf ye old interweb tubes.
So to figure out what part is broken you need to troubleshoot. To effectively troubleshoot you start at the bottom and work your way up testing each possibility until you find out what is not working. This is how you isolate the problem. Once you isolate the problem then you need to fix the problem.
Lets assume that this is your network configurations:
ip address: 192.168.0.10
gateway: 192.168.0.1
DNS address: 192.168.0.1
(if your using a firewall/router then dns address is often the same as your gateway, but that doesn't have to be true)
So this is how you do it in Linux.. Go step by step. It only works if you go in order.
If your using wireless then there are more steps to do if pinging your gateway fails...
1. Open up a terminal.
2. Run the 'ifconfig' command to determine if your ip address seems correct:
> ifconfig
If that is incorrect then dhcp is not working or you have a misconfiguration.
3. Ping your own address:
> ping 192.168.0.10
(hit ctrl-c to stop pinging)
If this is not working then you have serious issues.
4. Ping your gateway:
> ping 192.168.0.1
If tihs is not working then check the physical connections between your computer and the router/modem or whatnot. Use special wireless commands if your using wifi. Check the indicator lights on your nic card and router/modem to make sure that your connected correctly.
Could be driver issues or a misconfiguration. Come back here and give more information. Stuff like... the networking card or modem. The type and general configuration of the network. More details like that.
You can see the kernel output by using the 'dmesg' command. See if you can find something in there that looks like it may have to do with your networking device.
> dmesg | less
Also checking out your /var/log/messages file will be about the same thing.
5a. Ping your DNS server:
> ping 192.168.0.1
If this fails then the DNS server could be down or you have a incorrect IP address. If it's not the same as your gateway and you can't reach it then your router/firewall/modem is probably itself not connected to the internet. Check your indicator lights and configuration in your router.
5b. Ping a IP address on the internet:
> ping 64.233.167.104
If this fails then your modem or gateway is probably not connected correctly to the internet.
6. Ping a host on the internet:
> ping
www.google.com
If this fails then your DNS server is incorrectly configured. Either the server itself or in your /etc/resolv.conf configuration file.
You should have a entry that goes something like:
nameserver 192.168.0.1
7. Type
www.google.com into your browser.
If this fails and everything else works then probably your browser is broken for some reason.