i have a RealTek Semiconductor Ethernet card (RealTek RTL8168/8111 PCI-E GigabitEthernet NIC ),and D-LINK GLB-502T Modem
when i open System->Admin->Network
and activate my card..there it shows active but at the network applet(that is at the top-right side of the upper task bar) the symbol is disconnected(shows a cross sign in the network icon)..
when i right click that network icon and uncheck and recheck on "Enable netwok" the icon stays disconnected...and this message shows up - "the netwok has been disconnected"....
i use xDSL connection to connect to the internet...when i activate ppp0 along with eth0 at System->Admin->Network ,the internet doess'nt cnnct..

my gateway is 192.168.1.1
my ISP provides dynamic Ip address..to my internet cnnction
my ethernet IP is 192.168.1.2
Netmask - 255.255.255.0

You shouldn't need to activate PPP for ADSL connections. Disable it and just keep eth0 activated, you should have connectivity just with that.
After that, try pinging your gateway, a remote ip address like 208.67.222.222, and a domain like google.com. Which ones work?

yah...
but my ADSL is bridge connected
hence i have to dial my internet username and password to connect to the internet
hence i use ppp service..
previously i used ubuntu..
there used to be a command "pppoeconf" which determines my eth0 availability nad requires internet username and password (provided by my isp) to connect to the net..

Here my actual problem is probably my network card is never active..
when i try to activate(after a deactivation), ...this msg is shown "The network has been disconnected"

Check out /var/log/messages. The last few lines should show the logs of the card being activated. The part after that should show the error thrown when it gets deactivated. That should help narrow down the problem.

is SeLiniux messing with my network???
should i deactivate it???

Oct 8 17:07:05 localhost avahi-daemon[2356]: Joining mDNS multicast group on interface eth0.IPv4 with address 192.168.1.5.
Oct 8 17:07:05 localhost avahi-daemon[2356]: New relevant interface eth0.IPv4 for mDNS.
Oct 8 17:07:05 localhost avahi-daemon[2356]: Registering new address record for 192.168.1.5 on eth0.IPv4.
Oct 8 17:07:12 localhost pppd[3953]: pppd 2.4.4 started by root, uid 0
Oct 8 17:07:12 localhost pppd[3953]: Using interface ppp0
Oct 8 17:07:12 localhost pppd[3953]: Connect: ppp0 <--> /dev/pts/1
Oct 8 17:07:12 localhost pppoe[3954]: PPP session is 3726 (0xe8e)
Oct 8 17:07:16 localhost pppd[3953]: CHAP authentication succeeded: Authentication success,Welcome!
Oct 8 17:07:16 localhost pppd[3953]: CHAP authentication succeeded
Oct 8 17:07:16 localhost pppd[3953]: local IP address 59.93.204.186
Oct 8 17:07:16 localhost pppd[3953]: remote IP address 59.93.192.1


in this log my IP address is 59.93.204.186
but i actually i an bot connected to the internet
when i ping google the following shows up -

[root@localhost ~]# ping www.google.com
ping: unknown host www.google.com

Can you paste the output of ifconfig -a? Are eth0 and ppp0 up?