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Beginner

Really simple IPv6 question

Hi,

I know that this is simplifying things a lot, and ignore SLAAC etc, but:

Is IPv6 essentially the same as IPv4 but with much larger addresses and you nearly always use a public IP address?

By essentially the same I mean that in most situations you:

  1. Have a range of addresses allocated to you
  2. Have a DHCP server give these addresses out to your devices
  3. If you use VLANs you portion your address range up into smaller blocks and use subnets to distinguish between networks

I'm asking this because I've read a lot of conflicting answers to IPv6 questions.

Thanks

Andrew.

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Beginner

Really simple IPv6 question

Hi Andrew,

I chip in my 2 cents which do not pretend to be absolute truths .... or withstand the test of time

1. True these are usually a /64 prefix. If you have several networks usually providers allocate a /48 to you which is 65536 /64 prefixes;

2. possible but one of the advantages of IPv6 is to allow autoconfiguration. So if you configure the prefix (/64) in the router the hosts on that network will be able to to a Neighbor Discovery and receive the prefix and autoconfigure based on MAC address of interface

3. true but generally to allow 2 you use addresses with a /64 prefix therefore you ask for a /48 and the you have available a few thousands of network prefixes to assign to VLANs and remote LANs.

I am an home user with 4 VLANS and a remote network and they gave me a geographically routed /48.

No big deal and free.

Cheers

Fabio

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Re: Really simple IPv6 question

I am an home user with 4 VLANS and a remote network and they gave me a geographically routed /48.

Wow.  I thought normally ARIN only allocates /48 for ISPs, who break them down into /56 and /64 networks for their customers.  If I understand IPv6 addressing correctly, a /48 is 2^80 address, or 1,208,925,819,614,629,174,706,176 addresses (sombody check my math - is that right?).  Of course, considering that just to link two routers so IPv6 will route you need a /64, which is 2^64 or 18446744073709551616 addresses, maybe its no big deal.

Highlighted
Beginner

Re: Really simple IPv6 question

Wow that's a lot of addresses!

Thanks for clearing a few things up.

Sent from Cisco Technical Support iPad App

Highlighted
Beginner

Re: Really simple IPv6 question

John,

in IPv6 for autoconfiguration to work, the prefix has to be /64.

This is because the IPv6 host address is built from the MAC address which is 48 bits and insertint FF:FE between the higher and lower 3 bytes. As result you obtain 64 bits of host address and 64 bits of prefix. If the mac address is AA:AA:AA:BB:BB:BB the autoconfig IPv6 will be "prefix:AAAA:AAFF:FEBB:BBBB"

More detail about this you can read:

http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_7-2/ipv6_autoconfig.html

Also many times ISP are assigning a /64 to links:

in IPv4 we prefer to use "ip address unnumbered ethernet 0" command for point to point links like serials or to use RFC1918 address for those links in order to conserve address space. In IPv6 this is not necessary and a point to point link gets its /64 prefix for a network that will only have 2 hosts.

Not sure this is a good thing but they say we happen to have enough IPv6 addresses to provide one for each grain of sand on the planet ......

Fabio

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