problems about network settings

[loody]

Dear all:
I got 2 machines on the same switch.
Everything, except ip and distributions, one is mythubuntu and another
is Ubuntu with Gnome, is the same on these 2 machines, at least in my
mind.
But one can connect to the gatway but another cannot.
I don't think it is due to different distribution, but I have compare
everything I could on these 2 machines.
( I list ifconfig and route table at the end of letter)

is there any tool or command could help me to know where the problem is?
appreciate your help,
miloody

the setting of the fail one:
eth0      Link encap:Ethernet  HWaddr 00:11:25:d3:8c:11
          inet addr:172.20.87.209  Bcast:172.20.87.255  Mask:255.255.254.0
          inet6 addr: fe80::211:25ff:fed3:8c11/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:163 errors:0 dropped:0 overruns:0 frame:0
          TX packets:178 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:10919 (10.9 KB)  TX bytes:89622 (89.6 KB)
          Interrupt:16

$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
172.20.86.0     *               255.255.254.0   U     1      0        0 eth0
link-local      *               255.255.0.0     U     1000   0        0 eth0
default         172.20.87.254   0.0.0.0         UG    0      0        0 eth0

the setting of the passing one
eth0      Link encap:Ethernet  HWaddr 00:11:09:3f:c4:96
          inet addr:172.20.87.211  Bcast:172.20.87.255  Mask:255.255.254.0
          inet6 addr: fe80::211:9ff:fe3f:c496/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4504 errors:0 dropped:0 overruns:0 frame:0
          TX packets:327 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:384149 (384.1 KB)  TX bytes:49258 (49.2 KB)
          Interrupt:22
$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
172.20.86.0     *               255.255.254.0   U     1      0        0 eth0
link-local      *               255.255.0.0     U     1000   0        0 eth0
default         172.20.87.254   0.0.0.0         UG    0      0        0 eth0