C couldnt connect to the Internet while I'm connected to a network with internet?

I'm using a PC with Windows XP SP2 installed recently. I tried to connect with the network at my workplace and I found the DNS server ip and gave me the IP manaually. Then I successfully connect to the network (I can browse any computer on the network now). Now my problem is when I try to connect to the Internet. network cable works fine on my Mac computer to connect to the network and the Internet, but when I make the same cable to the PC of the network is working fine but no internet. There is no proxy server, so I left the proxy address in Internet Explorer empty. Is there any other settings I should do? I do not know the IP of the default gateway if that is the case. If so is there anyway I could find address of default gateway on my computer MAC (MAC is because my internet access from the same network.)

If u have to give the default gateway. if they can provide the IP address and subnet mask of the system then I can say ur ur default gateway u. You can get through some networks and subnets calculations

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