and if you want to be extra careful (or just plain pedantic :)...

some IP addresses while valid in form are not valid in practice. these are the martians and the unallocated.

the martians are reserved addresses that shouldn't be seen on the internet as a whole. but some of them can be used on local networks.

the unallocated are IP addresses that have not been allocated out of the global pool. these you shouldn't see at all (and shouldn't accept either).

you can find the current lists through a handy whois query.

$ whois -h whois.radb.net fltr-martian
filter-set: fltr-martian
filter:     {
            0.0.0.0/8^+ ,
            10.0.0.0/8^+ ,
            127.0.0.0/8^+ ,
            169.254.0.0/16^+ ,
            172.16.0.0/12^+ ,
            192.0.2.0/24^+ ,
            192.168.0.0/16^+ ,
            198.18.0.0/15^+ ,
            224.0.0.0/3^+
            }
descr:      Special use and reserved IPv4 prefixes.
remarks:    For the complete set of bogons, please see:
            fltr-unallocated - unallocated prefixes.
            fltr-bogons - fltr-unallocated + fltr-martian.
            http://www.cymru.com/Documents/bogon-list.html
admin-c:    Rob Thomas RT624
tech-c:     Rob Thomas RT624
notify:     radb@cymru.com
mnt-by:     MAINT-BOGON-FILTERS
changed:    radb@cymru.com 20021229
changed:    radb@cymru.com 20021230
changed:    radb@cymru.com 20021230
source:     RADB
...
[download]

do the same to find fltr-unallocated.