6to4 Addresses vs Regular IPv6 Addresses - Confusion

[TheFatalist]

Hi All, (Apologies if this question appears twice. Apparently I had an interface problem)

I'm finishing up my studies on the IPv6 portion of the CCNA material, and my book seems to completly ignore the fact that 6to4 addresses seem to look different than how the rest of the chapter portrays IPv6 addressing. I've done google searches, and unfortunately I don't have anyone at work who is versed enough in this who will know, so I'm hoping someone can help.

1. A normal IPv6 address:  2001:0db8:3c4d:0012:0000:0000:1234:56ab

   The same address in shorthand:  2001:db8:3c4d:12::1234:56ab

   A /64 of this space with autoconfiguration:  2001:db8:3c4d:12::/64 eui-64

   All of these addresses have, or will have 8 octets if written in long-hand, in other words - a valid IPv6 address.

Here's my problem:  The addresses in the 6to4 section look like this:

   ipv6 address 2001:db8:2:2::1/64

The book fails to mention what the "1" before the /64 is for, but I am assuming that it is an identifier and not part of the IPv6 address due to the " :: "   However, if this is the case, I only see 4 octets, and no eui-64 command, directing that the other 4 will be populated by the MAC padded.  And the "ipv6 address" syntax leads me to believe that this is a host address, and not a block/space statement (i.e. "ip address 192.168.0.1. 255.255.255.0," as opposed to "network 192.168.0.0 255.255.255.0)."

If someone could shed some light on this, it would be most appreciated, as it is very confusing.

[John.Ladwig]

What you're seeing is just another variant of the :: expansion rule.

2001:db8:2:2::1/64

expands to

2001:db8:2:2:0:0:0:1 with 64 bits of network.

Note that in the real world, 6to4 addresses always begin with 2002:, as the entire 2002::/16 space is granted to 6to4 operations.

2001:db82::/32 is allocated to documentation examples, and shouldn't be seen on live networks.

It takes a little practice to "see" v6 addresses quickly.  Less so if one is used to worling with CIDR address/prefix-length notation, rather more if one is customarily working with address mask notation.  v6 notation is always address/prefix-length - at least in every document or implementation I've yet come across.

    -jml

[Phillip Remaker]

Also, 6to4 IPv6 addresses have the IPv4 address of the 6to4 gateway embeded in them as well.  That is why they have a whole reseved block of 2002:/16.  Any gateway can algorithmically extract the IPv4 address for forwarding.

2002:0b0c:0205:5/48 for example, would be correlated to IPv4 address 11.12.2.5 (which is the decimal of the IPv4 address 0b0c:2025 shown in hex)

Earl Carter wrote a nice blog article on the general topic of IPv6 addresses at http://blogs.cisco.com/security/ipv6-addressing/

And the commonutiy has settled on calling the 16-bit numbers between the colons "hextets."  In IPv4, each 8-bit portion between dotrs is called an "octet."  The truly pedantic will object taht a hextet should be 6 dits, bit "hextet" is easier to say than "hexadecitet." :-)

Anotehr trivia point:  The letters in an IPv6 address should awalys be lower case.

More arcana can be found at http://tools.ietf.org/html/draft-hartmann-6man-addresspartnaming-01, which is on track to become an RFC.