root@ubuntu-server:~# nmap -sP 10.45.1.1-255

Starting Nmap 5.00 ( http://nmap.org ) at 2010-05-20 13:12 CEST
Host 10.45.1.1 is up.
Host 10.45.1.2 is up (0.0011s latency).
MAC Address: 00:19:E0:12:26:1E (Tp-link Technologies Co.)
Host 10.45.1.20 is up (0.040s latency).
MAC Address: 00:1C:C4:0C:66:5F (Hewlett Packard)
Host 10.45.1.30 is up (0.00093s latency).
MAC Address: 00:18:FE:C9:1F:80 (Hewlett Packard)
Host 10.45.1.31 is up (0.047s latency).
MAC Address: 00:14:A9:76:A2:40 (Cisco Systems)
Host 10.45.1.50 is up (0.0016s latency).
MAC Address: 00:04:13:24:D4:91 (Snom Technology AG)
Host 10.45.1.51 is up (0.0016s latency).
MAC Address: 00:0E:08:AB:BE:E5 (Cisco Linksys)
Host 10.45.1.253 is up (0.00036s latency).
MAC Address: 00:25:45:D0:08:D9 (Cisco Systems)
Nmap done: 255 IP addresses (8 hosts up) scanned in 1.91 seconds