我用Modem上网，连接好之后，用/sbin/ifconfig:
[work@localhost work]$ /sbin/ifconfig
lo        Link encapocal Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:10 errors:0 dropped:0 overruns:0 frame:0
          TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:840 (840.0 b)  TX bytes:840 (840.0 b)

ppp0      Link encapoint-to-Point Protocol
          inet addr:218.97.202.20  P-t-P:192.168.0.33  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:3070 errors:1 dropped:0 overruns:0 frame:0
          TX packets:3676 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:2072605 (1.9 Mb)  TX bytes:476762 (465.5 Kb)
开始我试着ping同一网络下其它机器：
[work@localhost work]$ ping -b 218.97.202.255 -c 5
PING 218.97.202.255 (218.97.202.255) 56(84) bytes of data.

--- 218.97.202.255 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4010ms
没有回应，我试着ping 知名网站：
[work@localhost work]$ ping www.sohu.com -c 5
PING pagegrp1.sohu.com (61.135.132.65) 56(84) bytes of data.

--- pagegrp1.sohu.com ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4010ms
也没有回应，我用host命令查找www.sohu.com的IP:
[work@localhost work]$ host www.sohu.com
www.sohu.com is an alias for pagegrp1.sohu.com.
pagegrp1.sohu.com has address 61.135.150.65
pagegrp1.sohu.com has address 61.135.150.67
pagegrp1.sohu.com has address 61.135.150.101
pagegrp1.sohu.com has address 61.135.150.113
pagegrp1.sohu.com has address 61.135.150.121
pagegrp1.sohu.com has address 61.135.150.145
pagegrp1.sohu.com has address 61.135.131.91
pagegrp1.sohu.com has address 61.135.131.180
pagegrp1.sohu.com has address 61.135.131.182
pagegrp1.sohu.com has address 61.135.131.183
pagegrp1.sohu.com has address 61.135.132.65
pagegrp1.sohu.com has address 61.135.132.80
pagegrp1.sohu.com has address 61.135.132.172
pagegrp1.sohu.com has address 61.135.132.173
pagegrp1.sohu.com has address 61.135.132.176
pagegrp1.sohu.com has address 61.135.145.47
均有结果，请问斑竹为什么ping不到？　　　　　　

ping不通是因为这些IP阻止了对ICMP包的回应，试试 baidu.com ，163.com
在本地做个测试： 用root执行
iptables -A INPUT -p icmp -j DROP
ping 218.97.202.20 看有什么结果，　　　　　　

ping本机和route没有问题了，但ping知名网站依然没有回应，我试着用traceroute来追踪：
[work@localhost work]$ host www.linuxeden.com
www.linuxeden.com has address 60.191.37.12
[work@localhost work]$ traceroute 60.191.37.12
traceroute to 60.191.37.12 (60.191.37.12), 30 hops max, 38 byte packets
1  192.168.0.5 (192.168.0.5)  121.722 ms  123.546 ms  117.711 ms
2  192.168.0.254 (192.168.0.254)  125.686 ms  115.599 ms  121.757 ms
3  218.97.192.69 (218.97.192.69)  124.042 ms  123.886 ms  119.942 ms
4  203.90.132.221 (203.90.132.221)  123.934 ms  124.006 ms  107.830 ms
5  203.90.132.253 (203.90.132.253)  127.945 ms  111.913 ms  111.923 ms
6  * * *
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只能进行部分追踪，这是为什么？　　　　　　

路由不全或者路由器上有错误的路由