News of Little Think About IPv6
This is one of my Personal note.
IPv6 addresses are 128-bit identifiers for interfaces and sets of
interfaces. There are three types of addresses:
Unicast:
An identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address.
Anycast:
An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to an anycast address is delivered to one of the interfaces identified by that address (the “nearest” one, according
to the routing protocols’ measure of distance).
Multicast:
An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to all interfaces identified by that address.
There are no broadcast addresses in IPv6, their function being superseded by multicast addresses.
In this document, fields in addresses are given a specific name, for example “subscriber”. When this name is used with the term “ID” for identifier after the name (e.g., “subscriber ID”), it refers to the contents of the named field. When it is used with the term “prefix” (e.g. “subscriber prefix”) it refers to all of the address up to and including this field.
In IPv6, all zeros and all ones are legal values for any field, unless specifically excluded. Specifically, prefixes may contain zero-valued fields or end in zeros.
Addressing Model
IPv6 addresses of all types are assigned to interfaces, not nodes. An IPv6 unicast address refers to a single interface. Since each interface belongs to a single node, any of that node’s interfaces’ unicast addresses may be used as an identifier for the node.
All interfaces are required to have at least one link-local unicast address (see section 2.8 for additional required addresses). A single interface may also be assigned multiple IPv6 addresses of any type (unicast, anycast, and multicast) or scope. Unicast addresses with scope greater than link-scope are not needed for interfaces that are not used as the origin or destination of any IPv6 packets to or from non-neighbors. This is sometimes convenient for point-to-point interfaces. There is one exception to this addressing model:
An unicast address or a set of unicast addresses may be assigned to multiple physical interfaces if the implementation treats the multiple physical interfaces as one interface when presenting it to the internet layer. This is useful for load-sharing over multiple physical interfaces.
Currently IPv6 continues the IPv4 model that a subnet prefix is associated with one link. Multiple subnet prefixes may be assigned to the same link.
Text Representation of Addresses
There are three conventional forms for representing IPv6 addresses as
text strings:
1. The preferred form is x:x:x:x:x:x:x:x, where the ‘x’s are the
hexadecimal values of the eight 16-bit pieces of the address.
Examples:
FEDC:BA98:7654:3210:FEDC:BA98:7654:3210
1080:0:0:0:8:800:200C:417A
Note that it is not necessary to write the leading zeros in an
individual field, but there must be at least one numeral in every
field (except for the case described in 2.).
2. Due to some methods of allocating certain styles of IPv6
addresses, it will be common for addresses to contain long strings
of zero bits. In order to make writing addresses containing zero
bits easier a special syntax is available to compress the zeros.
The use of “::” indicates multiple groups of 16-bits of zeros.
The “::” can only appear once in an address. The “::” can also be
used to compress the leading and/or trailing zeros in an address.
For example the following addresses:
1080:0:0:0:8:800:200C:417A a unicast address
FF01:0:0:0:0:0:0:101 a multicast address
0:0:0:0:0:0:0:1 the loopback address
0:0:0:0:0:0:0:0 the unspecified addresses
may be represented as:
1080::8:800:200C:417A a unicast address
FF01::101 a multicast address
::1 the loopback address
:: the unspecified addresses
3. An alternative form that is sometimes more convenient when dealing
with a mixed environment of IPv4 and IPv6 nodes is
x:x:x:x:x:x:d.d.d.d, where the ‘x’s are the hexadecimal values of
the six high-order 16-bit pieces of the address, and the ‘d’s are
the decimal values of the four low-order 8-bit pieces of the
address (standard IPv4 representation). Examples:
0:0:0:0:0:0:13.1.68.3
0:0:0:0:0:FFFF:129.144.52.38
or in compressed form:
::13.1.68.3
::FFFF:129.144.52.38
Text Representation of Address Prefixes
The text representation of IPv6 address prefixes is similar to the
way IPv4 addresses prefixes are written in CIDR notation. An IPv6
address prefix is represented by the notation:
ipv6-address/prefix-length
where
ipv6-address is an IPv6 address in any of the notations listed
in section 2.2.
prefix-length is a decimal value specifying how many of the
leftmost contiguous bits of the address comprise
the prefix.
For example, the following are legal representations of the 60-bit
prefix 12AB00000000CD3 (hexadecimal):
12AB:0000:0000:CD30:0000:0000:0000:0000/60
12AB::CD30:0:0:0:0/60
12AB:0:0:CD30::/60
The following are NOT legal representations of the above prefix:
12AB:0:0:CD3/60 may drop leading zeros, but not trailing zeros,
within any 16-bit chunk of the address
12AB::CD30/60 address to left of “/” expands to
12AB:0000:0000:0000:0000:000:0000:CD30
12AB::CD3/60 address to left of “/” expands to
12AB:0000:0000:0000:0000:000:0000:0CD3
When writing both a node address and a prefix of that node address
(e.g., the node’s subnet prefix), the two can combined as follows:
the node address 12AB:0:0:CD30:123:4567:89AB:CDEF
and its subnet number 12AB:0:0:CD30::/60
can be abbreviated as 12AB:0:0:CD30:123:4567:89AB:CDEF/60
Address Type Representation
The specific type of an IPv6 address is indicated by the leading bits
in the address. The variable-length field comprising these leading
bits is called the Format Prefix (FP). The initial allocation of
these prefixes is as follows:
Allocation Prefix Fraction of
(binary) Address Space
----------------------------------- -------- -------------
Reserved 0000 0000 1/256
Unassigned 0000 0001 1/256
Reserved for NSAP Allocation 0000 001 1/128
Reserved for IPX Allocation 0000 010 1/128
Unassigned 0000 011 1/128
Unassigned 0000 1 1/32
Unassigned 0001 1/16
Aggregatable Global Unicast Addresses 001 1/8
Unassigned 010 1/8
Unassigned 011 1/8
Unassigned 100 1/8
Unassigned 101 1/8
Unassigned 110 1/8
Unassigned 1110 1/16
Unassigned 1111 0 1/32
Unassigned 1111 10 1/64
Unassigned 1111 110 1/128
Unassigned 1111 1110 0 1/512
Link-Local Unicast Addresses 1111 1110 10 1/1024
Site-Local Unicast Addresses 1111 1110 11 1/1024
Multicast Addresses 1111 1111 1/256
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