ah ok
yeah it changes like every 24hours
Ahh...that would definitely explain it. The reason, of course, is for security. Before, the security of the cookies weren't exactly fool-proof.
If someone were able to get a copy of your PHP Session cookie or login and password cookies, they could effectively log in as you, which is dangerous. The new approach ties the Session with the IP as well as the cookies with the IP so that logging in as someone else is vastly more difficult. However, as is typical with increased security, that means the interface has become a little less friendly.
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It's all in the reflexes.