by Darius Kazemi, Jan 25 2019
RFC-25 is by Steve Crocker, dated 30 October 1969, the day after the first message was sent over ARPANET. It's titled “No High Link Numbers”.
The technical content
Here is the full content of this short RFC:
Because it may be desirable to reserve one or more link numbers for instrumentation purposes, and because 256 link numbers are many more than are needed, we suggest that no link number over 63 be used. At UCLA, we will implement our tables to take advantage of this limitation. We also note that 32 may be even more realistic, but 64 is certainly sufficient.
This is UCLA suggesting that people avoid reserving links 64 through 255 because who knows what future usage of the network might be and we might as well leave those open for future uses.
Note that this is a suggestion, not a specification! It's an example of the kind of early role RFCs played as a discussion mailing list before email was invented.