Needed help on server "Operation timed out." error

Hi folks, I am new to server building so I read through a lot of guides and tried to choose what exactly to do with it.
Now, I got a " Operation timed out." error when I tried to connect via lan, I successfully connected to it some time before I configure local DHCP to fix its lan address.
There are no errors in console, at least most of them are not even connected to this problem. There is a minor error for LWC auto update as it got a 502 error, but it is minor, and it was there when I was still able to connect it. For to check if its the server app's error, I connected via localhost (127.0.0.1) on that computer and it worked. I pinged the router on both computers, all fine, then when I ping from computer A to computer B, or from computer B to computer A, it just says
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1
Request timeout for icmp_seq 2
Request timeout for icmp_seq 3
ping: sendto: No route to host
Request timeout for icmp_seq 4
ping: sendto: Host is down
Request timeout for icmp_seq 5
ping: sendto: Host is down
Request timeout for icmp_seq 6
ping: sendto: Host is down
Request timeout for icmp_seq 7
I tried to restart the server, the entire computer, and also tried to move away the plugins on it. It didn't work. Note that i can still browse web from the server.

Server spec: ( a laptop, using wired connection, only.)
I3-2100
4GB DDR3
Ubuntu 12.04LTS
Bukkit 1.6.4 (latest) Dev build

So, yes, please kindly help me on this, thanks.