1. RFC 0025
Network Working Group                                      Steve Crocker
Request for Comments: 25                                            UCLA
                                                         30 October 1969

                           NO HIGH LINK NUMBERS

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
  1. RFC 0025