Why Linux can't visit public net?

[Edward King]

My computer's OS is Red Hat Linux7.3,I can ping this Linux computer from other computers,but in this Linux computer,I ping publish net,I fount I can't ping it,and I can't visit public net in Red Hat Linux7.3,for example:
[root at www root]# ping www.sina.com
ping: unknown host www.sina.com

Then I try ping gateway,I got follow result:
[root at www root]# ping 210.xx.xx.xx
PING 210.xx.xx.xx (210.xx.xx.xx) from 210.xx.xx.xx : 56(84) bytes of data.
>From 210.xx.xx.xx icmp_seq=1 Packet filtered
>From 210.xx.xx.x x icmp_seq=2 Packet filtered
>From 210.xx.xx.xx icmp_seq=3 Packet filtered
>From 210.xx.xx.xx icmp_seq=4 Packet filtered
>From 210.xx.xx.xx icmp_seq=5 Packet filtered

--- 210.xx.xx.xx ping statistics ---
5 packets transmitted, 0 received, +5 errors, 100% loss, time 4012ms

Then I try to ping DNS Server,I found I can't ping it!
[root at www root]# ping 210.x.x.x
connect: Network is unreachable

Then I use Windows OS,I configure IP address,gateway and DNS as same as Red Hat Linux7.3,I found I can ping public net and visit public net,like follows:
C:\Documents and Settings\Administrator>ping www.sina.com

Pinging us.sina.com.cn [66.77.9.79] with 32 bytes of data:

Reply from 66.77.9.79: bytes=32 time=190ms TTL=237
Reply from 66.77.9.79: bytes=32 time=181ms TTL=237
Reply from 66.77.9.79: bytes=32 time=190ms TTL=237
Reply from 66.77.9.79: bytes=32 time=181ms TTL=237

Ping statistics for 66.77.9.79:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 181ms, Maximum =  190ms, Average =  185ms

Why can't I  ping and visit public net under Red Hat Linux7.3? I am a beginer in Linux,please give me detail command.
Thanks in advance!

Best regards,
Edward
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