$_SERVER['REMOTE_ADDR'] in php reports different structure IP's from time to time.
What I'm used to seeing was something like XXX.XXX.XXX.XXX Now, I also have entries such as 2601:100f:b13b:4bde:63df:a4a3:dc92:c416 etc.
What is this? & how can I learn more about it? Is this part of the IPv6 thing which has to do with the fact they are running out of ip4 Range?
This is an IP address on the IPv6 format, add you have suggested. You can read more about this here... https://en.m.wikipedia.org/wiki/IPv6