小弟正在学习uboot，用的是mini2440的开发板，把国嵌提供的uboot下到norFlash中，启动uboot后用tftp下载内核映像到开发板中，但是下载后却启动不了，自己编译的内核和国嵌提供的内核均无法启动
tftp 31000000 uImage-nfs（下载内核映像，nfs根文件系统，下载成功了）
bootm 31000000 （启动内核）
内核是启动了，可是确失败了
Looking up port of RPC 100003/2 on 172.18.20.24
eth0: link up, 100Mbps, full-duplex, lpa 0x4DE1
rpcbind: server 172.18.20.24 not responding, timed out
Root-NFS: Unable to get nfsd port number from server, using default
Looking up port of RPC 100005/1 on 172.18.20.24
rpcbind: server 172.18.20.24 not responding, timed out
Root-NFS: Unable to get mountd port number from server, using default
Root-NFS: Server returned error -5 while mounting /nfsroot/rootfs
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or unknown-block(2,0)
Please append a correct "root=" boot option; here are the available partitions:
1f00 384 mtdblock0 (driver?)
1f01 2048 mtdblock1 (driver?)
1f02 128640 mtdblock2 (driver?)
1f03 131072 mtdblock3 (driver?)
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)
想了好几天也没想出来时怎么回事，请高人指点
我在环境变量中设置服务器ip为192.168.0.158，mini2440ip为192.168.0.200，但为什么提示 server 172.18.20.24 not responding，实在是困惑，请明白人指点一二
