Originally Posted by bubufr

There is no such thing as a “computer ID”. Furthermore, mobile phones are commonly served dynamic IP addresses, but those are private addresses. A large fraction of the mobile phones of a given operator actually share one public IP address, which means the IP seen from the Web sites do not really change as the mobile phone hops from cell tower to cell tower, but they are useless for tracking purposes since they are shared with many other users.

There are a lot of misconceptions on what airlines (or other online stores) can use to identify customers and offer different pricings. Theoretically, they can use all of the following:

• Cookies. This is the most natural and reliable manner to keep track of a user. It is certainly used for technical reasons to retain information about what searches a user has done and what offers to show him in priority. It is probably used by some airlines to change the pricing based on search history, but to my knowledge there has never been any definite proof of this. It is certainly not an overwhelmingly used practice. It is very much possible that pricing is affected by cookies for technical reasons that are not related to price information (e.g., based on the cookie information, another reservation is still being carried out by the user in another tab, so availabilities should be changed accordingly). Changing browsers, devices, or simply using private navigation mode suffices to clear the cookies.
• Magic strings in a URL. Sometimes used as an alternative to cookies, users are kept track of by remembering a token in the URL throughout the navigation to a Web site. Starting afresh from the main page of the Web site suffices to get rid of this tracking information.
• FFP information of the logged in user (this information is kept through cookies on one given browser, but it is obviously possible to log in on two different computers onto the same account). Same remarks as for cookies apply. Making sure one is not logged in suffices.
• Browser fingerprinting (see, e.g., https://panopticlick.eff.org/). These are elaborate techniques to identify a user’s browser without relying on cookies. This is used by online advertising software, especially when third-party cookies are blocked. This is rarely a perfect approximation. There is no reason to suspect that airlines use this, especially since they could use first-party cookies instead which are much more reliable.
• IP address. This is an utterly unreliable way of keeping track of a user: many users accessing the Internet do not have access to a public IP address, and therefore appear (through Network Address Translation) with the address of one central router of their company or ISP. In other situations, a given browser will switch IPs (e.g., when switching from one Wifi network to another) in a given browsing session. IP tracking is one of the big myth of airline sales. There is no reason to think that airlines would use this, when they can much more reliably use cookie tracking (and there is little evidence they even do use cookie tracking!). On the other hand, IP addresses are used for geolocating the user and selecting the market (country) of the sale. This has a definite impact on pricing, but it is often possible to select another market through the menus of the Web site.
• Other meta-information. The browser sends some other meta-information, such as the preferred languages, that are sometimes used to select the market. One other thing that may be of importance is the Referer information that allows a Web site to know where the user is coming from. This is commonly used by Web sites to offer rebates for customers coming from specific sites they are partnering with. Theoretically, it could be used to raise or lower the price of a flight based on which flight search engine a user is coming from. It is unclear whether this is common practice.

Finally, note that even if something is technically possible, it may not be legal. In particular, in Europe, arbitrary (i.e., with no objective reason) price discrimination between customers is generally not allowed. cf. in particular http://eur-lex.europa.eu/LexUriServ/...08E102:EN:HTML