May be it's just a bit miss configuration.
Try doing this...
$ping mail.yahoo.com ( yahoo.com is just an example, you may use another host)
if it said something like 'host not found' or 'unknown host' , try edit /etc/resolv.conf and add your dns server like this format
nameserver xxx.xxx.xxx.xxx
nameserver yyy.yyy.yyy.yyy
change x and y with your dns server.
if no errors found, it should return like this
PING login.yahoo.akadns.net (66.218.75.184) 56(84) bytes of data.
64 bytes from l1.login.vip.scd.yahoo.com (66.218.75.184): icmp_seq=1 ttl=238 time=252 ms
64 bytes from l1.login.vip.scd.yahoo.com (66.218.75.184): icmp_seq=2 ttl=238 time=247 ms
64 bytes from l1.login.vip.scd.yahoo.com (66.218.75.184): icmp_seq=3 ttl=238 time=257 ms
if you got error like 'Network is unreachable',
you may check your connection to your gateway server in your LAN,
try this command
$/sbin/route add default gw xx.yy.zz.nn ( change this with your gateway ip address)
then try the "ping" command above
if all this steps works and still can't make you get in to internet. Please let me know, and it will be great if you also include error message you got.