Suppose that I connected to internet -- from ADSL modem or wireless -- is a command line for linux for PC not server or cisco switch that I can detect my partner (we have a modem 4 port LAN example) network in that service is used, for example i use net, my friend use too, I want to detect neighbor computers that use net and if is possible I can send data to his?
Since you are all on the same LAN, yes you can send data to other computers.
There is no specific command to detect them, but there are a number of things you can do to find them.
Since all the computers use DHCP to get addresses, the odds are very high that your "partner" computers are numerically close to you. You can simply ping addresses near yours and wait for a reply. For example, if your address is 192.168.0.4, and your gateway is 192.168.0.1, chances are that .2, .3 and possibly .5 are all used by other computers on your LAN.
If the computers are Windows boxes, they will find each other to support local file sharing. You can then look in your ARP cache for their addresses using the command
Some machines will have a firewall enabled, so they might not respond to pings. But all have some open ports, so using a tool like NMAP will help you find them.