Decimal
IP address is a form of internal representation for the IP address. It's formed by taking the four numbers of IP in
dotted quad form, interpreting them as binary numbers, putting them into 4 bytes and interpreting them as 4-byte
integer (in
network byte order, I guess).
For example, the decimal IP of 255.255.255.255 is 354267876296.
IRC uses these internally, as do some other protocols, and naturally some people store IPs this way.
Also, spammers seem to favor this representation of IP addresses (never mind that some browsers can't hack decimal IPs at all as part of URLs because the Specifications don't require it).
In UNIX, these are handled with inet_ntoa() and inet_aton().
To convert decimal IPs to normal dotted quad form (for example, to spoil spammers' joy), use this Perl hack:
perl -MSocket -e "print inet_ntoa(pack('N','354267876296'));"
Replace number accordingly.