In reply to:

When referencing metric sizing of automobile engines, I say " it has a 5point7 litre engine....", written 5.7

As do I... it's a decimal point. The 350cid engine roughly translates to 5 litres plus 7/10th of a litre.
In reply to:

When referencing TCP/IP addresses, I say "10dot10dot15dot150" written 10.10.10.150

I usually leave the dots out. Just "one nine two... one six eight... sixty-nine *heheh heheheheheh heheh* I do use the term "dot" when explaining to an inexperienced user how to enter an IP address.

Bren R.