Editing Teredo tunneling (section)

=== IPv6 addressing ===
Each Teredo client is assigned a public [[IPv6 address]], which is constructed as follows (the higher order bit is numbered 0):
* Bits 0 to 31 hold the Teredo prefix (2001::/32).
* Bits 32 to 63 embed the primary IPv4 address of the Teredo server that is used.
* Bits 64 to 79 hold some flags and other bits; the format for these 16 bits, MSB first, is "CRAAAAUG AAAAAAAA".  The "C" bit was set to 1 if the Teredo client is located behind a [[Network address translation#Methods of translation|cone NAT]], 0 otherwise, but RFC 5991 changed it to always be 0 to avoid revealing this fact to strangers. The "R" bit is currently unassigned and should be sent as 0.  The "U" and "G" bits are set to 0 to emulate the "Universal/local" and "Group/individual" bits in [[MAC address]]es.  The 12 "A" bits were 0 in the original RFC 4380 specification, but were changed to random bits chosen by the Teredo client in RFC 5991 to provide the Teredo node with additional protection against IPv6-based scanning attacks.
* Bits 80 to 95 contain the ''obfuscated'' UDP port number. This is the port number that the NAT maps to the Teredo client, with all bits inverted.
* Bits 96 to 127 contain the ''obfuscated'' IPv4 address. This is the public IPv4 address of the NAT with all bits inverted.

{{center|''Teredo IPv6 addressing table''}}
{| class="wikitable" style="margin-right:auto; margin-left:auto;"
|-
! Bits || 0 - 31 || 32 - 63 || 64 - 79 || 80 - 95 || 96 - 127
|-
| Length || 32 bits || 32 bits || 16 bits || 16 bits || 32 bits
|-
| Description || Prefix || Teredo<br />server IPv4 || Flags || Obfuscated<br />UDP port || Obfuscated Client<br />public IPv4
|}

As an example, the IPv6 address 2001:0000:4136:e378:8000:63bf:3fff:fdd2 refers to a Teredo client that:
* Uses Teredo server at address 65.54.227.120 (4136e378 in [[hexadecimal]])
* Is behind a cone NAT and client is not fully compliant with RFC 5991 (bit 64 is set)
* Is probably (99.98%) not compliant with RFC 5991 (the 12 random bits are all 0, which happens less than 0.025% of the time)
* Uses UDP mapped port 40000 on its NAT (in hexadecimal [[Bitwise NOT|not]] 63bf equals 9c40, or decimal number 40000)
* Has a NAT public IPv4 address of 192.0.2.45 (not 3ffffdd2 equals c000022d, which is to say, 192.0.2.45)

{{center|''Teredo IPv6 example table''}}
{| class="wikitable" style="margin-right:auto; margin-left:auto;"
|-
! Bits || 0 - 31 || 32 - 63 || 64 - 79 || 80 - 95 || 96 - 127
|-
| Length || 32 bits || 32 bits || 16 bits || 16 bits || 32 bits
|-
| Description || Prefix || Teredo<br />server IPv4 || Flags || Obfuscated<br />UDP port || Obfuscated Client<br />public IPv4
|-
| Part || 2001:0000 || 4136:e378 || 8000 || 63bf || 3fff:fdd2
|-
| Decoded || || 65.54.227.120 || cone NAT || 40000 || 192.0.2.45
|}