I am getting messages like this with bitcoind.  I understand that this is an indication of a valid proof of work.  The part I do not get is the "26 of the last 100 blocks have unexpected version"

2018-12-02T11:59:36Z UpdateTip: new best=00000000000000000025f33f934ce1f4a25899468b945107d0e8761bcab05e17 height=552263 version=0x20000000 log2_work=90.085067 tx=361390465 date='2018-12-02T11:59:18Z' progress=1.000000 cache=3.8MiB(28145txo) warning='26 of last 100 blocks have unexpected version'


Is this common?  Is this an indication of "extra nonce"?  Is there an online reference to aceptable strategies of extra nonce?

As my name indicates, I am new.  Even basic information would be appreciated.

Thank you for your reply.

I also found out about bitcoin-cli from the Mastering Bitcoin book you recommended and I had on order (I did not mention the title exact title).  It appears that bitcoin-cli needs bitcoind runnning.

It looks like https://sourceforge.net/projects/cpuminer/ might be the reference I need for CPU mining.  I am sure that once I build it, forking it off will be no problem.

I am very new to bitcoin.  I have only read an introductory  book on the technology and I have an Oriley book on bitcoin on order.  I am looking for a manual or guide that describes the standard interfaces to to the blockchain and transaction information so that I can try to design my own mine (for fun only of course) 

Any help would be appreciated.