Merge pull request #1531 from sterlingw/master

Fix book link

free-programming-books.md

@@ -493,7 +493,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [HTTP Succinctly, Syncfusion](http://www.syncfusion.com/resources/techportal/ebooks/http) (PDF, Kindle) *(Just fill the fields with any values)*
 * [HTTP2 Explained](http://daniel.haxx.se/http2/) - Daniel Stenberg
 * [The TCP/IP Guide](http://www.tcpipguide.com/free/t_toc.htm)
-* [Understanding IP Addressing: Everything you ever wanted to know](http://www.apnic.net/__data/assets/pdf_file/0020/8147/501302.pdf) (PDF)
+* [Understanding IP Addressing: Everything you ever wanted to know](http://www.di.unipi.it/~ricci/501302.pdf) (PDF)
 * [ZeroMQ Guide](http://zguide.zeromq.org/page%3Aall)