Is it OK to use shorter ULA-prefix?

[openwrtforever]

I use fd00::/48 so that it easy to remember and open devices directly for example fd00::3, I wanted to make sure there is nothing wrong with it since it needs to be unique but this one is very common and easy. Just making sure it does not interfere with the devices on the Internet.

[vgaetera]

What you want should be possible but goes against RFC:

https://tools.ietf.org/html/rfc4193

The allocation of Global IDs is pseudo-random RANDOM. They MUST NOT be assigned sequentially or with well-known numbers. This is to ensure that there is not any relationship between allocations and to help clarify that these prefixes are not intended to be routed globally. Specifically, these prefixes are not designed to aggregate.

At least it would be wrong to call it a ULA prefix.

[hnyman]

@openwrtforever

Just making sure it does not interfere with the devices on the Internet.

Those addresses should not be not routed outward, so supposedly your well-behaving ISP would not route that "private" address forward even if you would leak something toward them. So it should cause no harm to global internet.

ULA addresses are not supposed to be guaranteedly unique. Quite similar as 192.168.x.x or 10.x.x.x in IPv4. Local LAN thing, not routable to public internet.

Problems might arise in case if also your ISP uses similar non-standard crude simplification of zeros in its IPv6 network, so that your router would have link-local fd00:0000:0000:: addresses on both WAN and LAN sides

Ps.

@openwrtforever

Is it OK to use shorter ULA-prefix?

Your question should probably be:
" Is it OK to use fd00:0000:0000:: as ULA-prefix? "

It is not about shortness, but about non-random zeros making it possibly non-unique.