The prefix 128.0/16 is filtered in Juniper devices up to and including JUNOS software version 11.1. We looked at three ways to get a rough estimate on how much filtering of 128.0/16 is going on on the Internet.

Some IP address blocks are more special then others; RFCs contain a lot of exception cases, and worse, these exceptions sometimes come and go. The address block 128.0/16 is one of these cases. It was initially reserved by the IANA, but in RFC3330 (September 2002) this block was let go to the free pool. From RFC3330: - This block, corresponding to the numerically lowest of the former Class B addresses, was initially and is still reserved by the IANA. Given the present classless nature of the IP address space, the basis for the reservation no longer applies and addresses in this block are subject to future allocation to a Regional Internet Registry for assignment in the normal manner.

This was further obsoleted by RFC 5735 (January 2010), which doesn’t mention 128.0/16 at all.

As became apparent recently, in this thread on the nsp-juniper mailinglist, Juniper devices running JUNOS versions up to 11.1 still have 128.0/16 listed as a martian by default, which means any device running JUNOS with this default will ignore routing information about this address block. This thread also contains pointers on how to work around or fix the problem.

We wanted to know what the scale of the problem was so we assessed this in three different ways.