255.255.255.255/0 would scan
ALL IPv4 addresses (Included LAN, reserved, special, etc...), although, I don't recommend you do it, your ISP will get abuse complaints and they'll forward that onto you.
Quick example (Ctrl+c'd it before it got to any real hosts):-
Code:
Nmap scan report for 0.0.0.1 [host down]
Nmap scan report for 0.0.0.2 [host down]
Nmap scan report for 0.0.0.3 [host down]
Nmap scan report for 0.0.0.4 [host down]
Nmap scan report for 0.0.0.5 [host down]
Nmap scan report for 0.0.0.6 [host down]
Nmap scan report for 0.0.0.7 [host down]
Nmap scan report for 0.0.0.8 [host down]
Nmap scan report for 0.0.0.9 [host down]
Nmap scan report for 0.0.0.10 [host down]
Nmap scan report for 0.0.0.11 [host down]
Nmap scan report for 0.0.0.12 [host down]
Nmap scan report for 0.0.0.13 [host down]
Nmap scan report for 0.0.0.14 [host down]
Nmap scan report for 0.0.0.15 [host down]
Nmap scan report for 0.0.0.16 [host down]
Nmap scan report for 0.0.0.17 [host down]
Nmap scan report for 0.0.0.18 [host down]
Nmap scan report for 0.0.0.19 [host down]
Nmap scan report for 0.0.0.20 [host down]
Nmap scan report for 0.0.0.21 [host down]
Nmap scan report for 0.0.0.22 [host down]
Nmap scan report for 0.0.0.23 [host down]
Nmap scan report for 0.0.0.24 [host down]
Nmap scan report for 0.0.0.25 [host down]
Nmap scan report for 0.0.0.26 [host down]
Nmap scan report for 0.0.0.27 [host down]
Nmap scan report for 0.0.0.28 [host down]
Nmap scan report for 0.0.0.29 [host down]
Nmap scan report for 0.0.0.30 [host down]
Nmap scan report for 0.0.0.31 [host down]
Nmap scan report for 0.0.0.32 [host down]
Nmap scan report for 0.0.0.33 [host down]
Nmap scan report for 0.0.0.34 [host down]
Nmap scan report for 0.0.0.35 [host down]
Nmap scan report for 0.0.0.36 [host down]
Nmap scan report for 0.0.0.37 [host down]
Nmap scan report for 0.0.0.38 [host down]
Nmap scan report for 0.0.0.39 [host down]
Nmap scan report for 0.0.0.40 [host down]
Nmap scan report for 0.0.0.41 [host down]
Nmap scan report for 0.0.0.42 [host down]
