centos5.1不能上网的解决方法，有需要的朋友可以参考下。
案例环境：
1.双系统：xp和centos5.1
2.在xp下可以上网，centos5.1不能上网
3.centos5.1中，不能ping通网关。
4.xp中网卡设置为：
ip：192.168.1.57
掩码：255.255.255.0
网关：192.168.1.10
DNS：192.168.1.10
在centos5.1中查看网卡信息，显示如下：
[root@localhost ~]# ifconfig -a
eth0 Link encap:Ethernet HWaddr 00:E0:4C:2A:2E:0C
inet addr:192.168.1.57 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::2e0:4cff:fe2a:2e0c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1511 errors:0 dropped:0 overruns:0 frame:0
TX packets:304 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:139496 (136.2 KiB) TX bytes:32922 (32.1 KiB)
查看网关，显示如下信息：
[root@localhost ~]# vi /etc/sysconfig/network-scripts/ifcfg-eth0
查看DNS，显示如下信息：
[root@localhost ~]# cat /etc/resolv.conf
search localdomain
可以发现，原来网关和DNS不对，更改后重启网络服务：
[root@localhost ~]# service network restart
Shutting down interface eth0: [ OK ]
Shutting down loopback interface: [ OK ]
Disabling IPv4 packet forwarding: net.ipv4.ip_forward = 0
[ OK ]
Bringing up loopback interface: [ OK ]
Bringing up interface eth0: [ OK ]
[root@localhost ~]# ping 192.168.1.10
PING 192.168.1.10 (192.168.1.10) 56(84) bytes of data.
64 bytes from 192.168.1.10: icmp_seq=1 ttl=64 time=0.475 ms
64 bytes from 192.168.1.10: icmp_seq=2 ttl=64 time=0.324 ms
--- 192.168.1.10 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.324/0.399/0.475/0.078 ms
[root@localhost ~]# ping www.baidu.com
PING www.a.shifen.com (119.75.217.56) 56(84) bytes of data.
64 bytes from 119.75.217.56: icmp_seq=1 ttl=54 time=22.8 ms
64 bytes from 119.75.217.56: icmp_seq=2 ttl=54 time=25.3 ms
64 bytes from 119.75.217.56: icmp_seq=3 ttl=54 time=25.2 ms
--- www.a.shifen.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1998ms
rtt min/avg/max/mdev = 22.829/24.474/25.332/1.170 ms