Show all IP addresses on your LAN ? I don't think I've understood properly.. there is no command that will show you that... you could open Network Neighborhood and see all the machines that are in your LAN..
otherwise you would have to get yourself a mass-ping utility that will consecutively ping all IP addresses in a range.. I would recommend nmap
or superscan (search for it)
You could even try Sam Spade.
Sahir's right, there is no Windows command available that will show you all active hosts within a given network.
Alternative you can download IP Network Browser (Downloads>Network Security & Administration>More Network Tools). I personally have been using this application for years - its fast and stable, supporting SNMP aswell - should you have any hosts that run a SNMP server.
Assuming you want to find only all the Windows machines on your network do the following :
 Wait for latest worm to come out (should be something on one of the ASN.1 vulnerabilities within a couple of months)
 Don't patch your systems
 Watch all your Windows boxen announce themselves all over the network
hehe I'm just kidding. Use the information from one of the two posts above.