Good question! This is going to be a bit long, so bear with me

This is a distillation of the results of 230+ comments on #515, not to mention the 300+ comments spread across several other older issues that fed into that one. I know it's long. Please don't complain unless you can offer a shorter write-up. :-)

I appreciate the time and effort you put into your answer, but frankly this is still completely incomprehensible to me.

thank you again for taking the time to explain how the compiler thinks in such an elaborate way. This is some blog post material!

Thank you for this answer, perfectly detailed.

I've done some extensive research and experimentation. Here are my results.

thank you for the very detailed explanation of the issue

this explanation is not gonna be succinct, but I couldn't express this clearly, sorry!

In depth

The Analytics JavaScript Tag When a JavaScript-enabled web browser loads a page with the Analytics tag (ga.js or analytics.js), it does two things asynchronously: load and process the Analytics function queue and request the Analytics JavaScript. The function queue is a JavaScript array where the different Analytics configuration and collection functions are pushed. These functions, which are set by the site owner when implementing Analytics can include functions like specifying the Analytics account number and actually sending page view data to the Analytics Collection Network for processing. When the Analytics JavaScript runs a function from the function queue that triggers data to be sent to the Analytics Collection Network (this function is typically ga('send', 'pageview') in the analytics.js JavaScript library and _trackPageview in the ga.js library), it sends the data as URL parameters attached to an HTTP request for http://www.google-analytics.com/_utm.gif (for ga.js) and http://www.google-analytics.com/collect (for analytics.js). If the anonymization function has been called prior to the page tracking function, an additional parameter is added to the pixel request. The IP anonymization parameter looks like this: &aip=1 The Analytics Collection Network The Analytics Collection Network is the set of servers that provide two main services: the serving of ga.js and analytics.js (the Analytics JavaScript) and the collection of data sent via requests for _utm.gif and /collect. When a request for ga.js, analytics.js, _utm.gif, or /collect arrives, it includes additional information in the HTTP request header (i.e. the type of browser being used) and the TCP/IP header (i.e. the IP address of the requester). As soon as a request for _utm.gif arrives, it is held in memory for anonymization. If the &aip=1 parameter is found in the request URL (as it would have been placed by the Analytics JavaScript after processing the anonymization function in ga.js or analytics.js ), then the last octet of the user IP address is set to zero while still in memory. For example, an IP address of 12.214.31.144 would be changed to 12.214.31.0. (If the IP address is an IPv6 address, the last 80 of the 128 bits are set to zero.) Only after this anonymization process is the request written to disk for processing. If the IP anonymization method is used, then at no time is the full IP address written to disk as all anonymization happens in memory nearly instantaneously after the request has been received.