Mobile IP
IPv6-to-IPv4 transition mechanismsThere are different types of IPv6 addresses. Unicast addresses are the well-
known addresses; packets sent to these addresses arrive directly at the
interface that belongs to the address. Anycast addresses look the same as
unicast addresses, but they actually address a group of interfaces; packets sent
to an anycast address arrive at the nearest (in the router metric sense)
interface. Anycast addresses may only be used by routers. Finally, a multicast
address identifies a group of interfaces; packets sent to a multicast address
arrive at all interfaces belonging to the multicast group.
IPv6 addresses are created using eight sets of numbers, like this:
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F734:0000:0000:0000:3458:79B2:D07B:4620
Each of the eight sections is made of a four-digit number in hexadecimal,
which means that each digit can be from 0 to 9 or A to F (A=10, B=11, and so
on). Hexadecimal is a denser format than binary. In binary, there are only two
options, 0 and 1. This means that in hexadecimal, 4 digits can be used to
represent 16 binary digits, like this:
Binary 0000000000000000 = hex 0000 (or just 0)
Binary 1111111111111111 = hex FFFF
Binary 1101010011011011 = hex D4DBSo, a 128-bit address written in binary would be very long indeed. This 128-
bit address written in binary and separated by dots:
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1111111111111111.1111111111111111.1111111111111111.1111111111111111.11111111111
11111.1111111111111111.1111111111111111.1111
is the same as this 128-bit address, written in hexadecimal and separated by
colons:
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FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF
So, understandably, we use the latter hexadecimal format for IPv6 (and the
binary format is not used, just in case you were wondering).
Often an address has long substrings of all zeros; the longest and first run of
all-zero sections is abbreviated as a double colon (::). Because :: is
variable in length, it can be used only once per address. Leading 0s—up to