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IPv6 was originally designed because the number of available unregistered IPv4 addresses was running low. Because IPv6 uses a 128-bit addressing scheme, it has more than 79 octillion (that’s 79,000,000,000,000,000,000,000,000,000 to you and me) times as many available addresses as IPv4. Also, instead of using binary digits or decimal digits, IPv6 uses eight sets of four hexadecimal digits, like so:


In addition, you can abbreviate these very long addresses by dropping leading zeros (like the zero before the B in “0B00”). You can also drop any single grouping of zero octets (as in the number above) between numbers as long as you replace them with a double colon (::) and they are complete octets (you can’t drop the three zeros in the second octet to make it just “B” instead of “0B00,” for example). If you apply this rule (known as the zero compression rule) to the above address, it would make the example address look like so:


You can’t use the zero compression rule to drop more than one grouping of zero octets. For example, you can’t make 3FFE:0000:0000:0002:0000:0000: 0000:000C into 3FFE::0002::000C. This is also part of the zero compression rule: There can be only one set of double colons!

As with IPv4, there are several addresses that are reserved for special uses. The IPv6 address ::/0 is the default address for a host (like in IPv4). The address ::1/128 is reserved for the local loopback (like in IPv4). IPv6 also includes provisions for the old IPv4 hosts so they can be migrated to the new addressing scheme. This is accomplished by using the address where the last four sets of digits refer to the old IPv4 address.

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