# IPV6: How Many IP Addresses Can Dance on the Head of a Pin?

-March 28, 2008

Because I serve as an occasional docent at the Computer History Museum, I get messages from the other docents. These messages usually contain some pretty interesting and arcane information about particular exhibits or other computer topics. I just got this one on the “new” IPV6 Internet addressing scheme from Dick Guertin and thought I’d pass it along (Note: The IMP is the ARPANET Interface Message Processor, based on a Honeywell DDP-516 minicomputer):

When I get to the IMP, I tell people about the Internet and IP addresses.  When first invented, they were 4-byte values, represented by four decimal numbers separated by dots, like 171.64.19.80, etc. The decimal numbers are 0 thru 255 (256 possible values per number). That’s because they are held in bytes which have a decimal limit of 255.  When combined, they yield 232 possible values, or about 4-billion values.  They never thought they’d run out.

BUT, there are 6-billion people on the planet, so if everyone was assigned just one IP address, we’d run out and leave 1/3rd of the world without IP addresses.

So they invented IPV6, a 128-bit value, which is 16-bytes long. Since they had to identify this to distinguish it from 4-byte values, the 1st byte has a 1-byte value that was never used in the 1st byte of the original 32-bit addresses.  So that leaves 2120 possible IP addresses using IPV6.

How big is that?  Well, several web sites say there are 1.33 x 1050 atoms in the earth.  That’s way bigger than 2120.  But to make it come closer, I computed the number of atoms on the surface of the earth.  That turns out to be 1.26 x 1034 atoms.  2120 is 1.33 x 1036, which is still bigger by 105 times.

So we could assign an IPV6 address to EVERY ATOM ON THE SURFACE OF THE EARTH, and still have enough addresses left to do another 100+ earths.  It isn’t remotely likely that we’ll run out of IPV6 addresses at any time in the future