An IP address is a 32 bit number. It is generally written in the "dotted quad" notation: w.x.y.z. To convert an IP address to base 10, calculate w*16777216+x*65536+y*256+z. Or use the following Perl code, which takes as an argument as a string in dotted quad form:

sub dqtoint {
return hex ('0x' .
join '',
(map {
sprintf '%lx', $_
split (/\./, shift)));

m_turner suggests using bc to convert back:
< Enter your number here >

The output will be your ip address in dotted quad for (but without dots). m_turner further suggests to do the equivalent of my Perl above:
< enter your number here in dotted quad form without dots >