A website can be contacted by just a number? yes it can be, which is to the base 10.
The conversion is very simple like if your IP address is 10.20.30.40. The number that works is [10*(256*256*256)]+[20*(256*256)]+[30*256]+40. Just replace the sum with the number you get and see it works...
Such numbers can found directly at Network Tools Website.
Internet Assigned Numbers Authority (IANA) works to assign IP Addresses to individuals on internet. The series has to be purchased and then used. A person connected to internet either uses the purchased IP Address or the one from ISP. The assignment is based on a RFC2050
Soon this IP address will be changed to ' IPv6 ' which will provide more number IP addresses all over th world. This will overcome the limitation of number of simultaneous users from 2^32 to 2^64.