Weebly's IP Addresses are 188.8.131.52, 184.108.40.206, 220.127.116.11, 18.104.22.168 and 22.214.171.124; you can use whichever of these you feel like using. Any one of these will work, but if you find this confusing, just use the 126.96.36.199 IP. You should also see a message including the IP address if you publish your site to your domain name before changing the A Records at 1&1.
There are a couple of things to note with regard to 1&1 domains:
The Weebly IP address should be entered into the IPv4 field, and if there are any IPv6 records set for your domain, you'll need to delete those. If you have any difficulty doing this, please reach out to 1and1 support - they'll be able to get you up and running quickly!