ipv6 – Why the FF:FE in EUI-64?

I just lately had an analogous query and researching that matter made me perceive it fairly properly and I really feel like I must also share the main points associated to this query right here.

Terminology

It is essential to know what we’re working with since I typically see these phrases used incorrectly.

EUI-64: It is a 64 bit identifier described by IEEE 802 which can be created from a MAC-48 or an EUI-48 handle. (Extra particulars concerning the conversion later.)

Modified EUI-64: Is an EUI-64 handle with it is u/l bit inverted. That is described in RFC 4291 and might function an IPv6 interface identifier.

Codecs

To know why FFFE₁₆ is used we first have to grasp the construction of the addresses, which you’ll see beneath:

c - hex digit, company_id
e - hex digit, extension-identifier

EUI-48 format (OUI-24)
cc cc cc ee ee ee

EUI-64 format (OUI-24)
cc cc cc ee ee ee ee ee

EUI-64 format (OUI-36)  
cc cc cc cc ce ee ee ee        

From this we will see that in EUI-64 corporations might assing identifiers within the vary 0-FFFFFFFFFF₁₆ if they’ve a 24 bit OUI or 0-FFFFFFF₁₆ if they’ve a 36 bit OUI.

Nevertheless to assist unabigous encapsulation of EUI-48/MAC-48 addresses in EUI-64 IEEE forbids producers to make use of identifiers in the long run of their assigned vary, so the subsequent codecs are by no means assigned:

EUI-48 Extension
OUI-24 cc cc cc FF FE ee ee ee
OUI-36 cc cc cc cc cE ee ee ee

MAC-48 Extension
OUI-24 cc cc cc FF FF ee ee ee
OUI-36 cc cc cc cc cF ee ee ee

So the truly usable handle ranges are 0-FFFDFFFFFF₁₆ and 0-DFFFFFF₁₆ this manner the remaining area can be utilized to encapsulate 48 bit addresses.

Supply: Tips for 64-bit World Identifier (EUI-64™) Registration Authority

Notice: IPv6 ought to’ve used FFFF₁₆ however they misunderstood the distinction between MAC-48 and EUI-48 and since this does not trigger points they only stored it. (That is additionally famous within the RFC.)