# IPv6 EUI-64 explained

**URL:** https://forum.networklessons.com/t/ipv6-eui-64-explained/1176
**Category:** Lessons Discussion
**Created:** [December 27, 2016, 11:30pm UTC](https://forum.networklessons.com/t/ipv6-eui-64-explained/1176 "2016-12-27T23:30:35Z")
**Posts on this page:** 1
**Showing post:** 16

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### Author: ![lagapidis](https://cdn-forum.networklessons.com/user_avatar/forum.networklessons.com/lagapidis/32/4949_2.png) [@lagapidis](https://forum.networklessons.com/u/lagapidis)
#### Post date: [June 28, 2018, 10:14am UTC](https://forum.networklessons.com/t/ipv6-eui-64-explained/1176/16 "2018-06-28T10:14:55Z")

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Hello Nadav

This is an excellent question. According to [RFC 5375](https://tools.ietf.org/html/rfc5375) in section **B.2.4 EUI-64 ‘u’ and ‘g’ Bits** , it states the following:

> When using subnet prefix lengths other than /64, the interface  
> identifier cannot be in Modified EUI-64 format as required by  
> [RFC4291]. However, nodes not aware that a prefix length other than  
> /64 is used might still think it’s an EUI-64; therefore, it’s prudent  
> to take into account the following points when setting the bits.

The document continues to state several considerations that should be checked carefully. In general, depending primarily on the vendor, using a prefix other than /64 may be possible in conjunction with EUI-64 IPv6 address computation, however, because it may have unpredictable results, it’s a good idea not to implement it.

I hope this has been helpful!

Laz

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