I thought that it was because of two machines on the network having the same hostname, and hence asked this question. But it did not resolve the problem. The machine responds to my pings. But when I go to Places > Network, I can't view that machine. I also can't view the other machines on the network from his machine. What seems to be the problem?
EDIT: I can view the folders using the samba protocol, by typing this in nautilus:
smb://192.168.1.5
But I cannot access the tomcat running on his machine, by typing this in the browser:
http://192.168.1.5:8000
There is a mac-mini in my office, and that can view this machine, and vice-versa.
Linux default do not allow access in network neighbourhood - in this window You will see only SMB hosts which mean: windows, or Linux with samba enabled and running. So probably You do not define smb shares on Your machines - define some shared directories ( in Gnome right context menu on folder - option "share this directory" or so).
Viewing the computer in Places > Network is based on a different protocol. Ping is a very low level debug tool. It does not scan the network and build a list of other computers on the network.
SMB (Windows File Sharing) is a set of protocols and programs that communicate to provide that list of what computers are on the network and what folders they are sharing. Along with sharing files itself.
By default (and unlike windows), Ubuntu does not come with SMB (Called Samba in Linux) turned on by default. A Windows box will not see a Ubuntu box in Network Neighborhood (or Network Places, or whatever Microsoft is called that now days), if Samba is not installed and turned on in Ubuntu.
In Ubuntu 10.04 it's real easy to do. Take a look at this guide. http://www.simplehelp.net/2007/05/19/how-to-share-files-and-folders-in-ubuntu/
Once you have something set to shared, other computers will then see that computer on the network.