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According to your link: "Personal data relating to GDPR does not cover: ... Properly anonymized data ..."

So OP might could hash the ip address (something better than md5, ideally) and save that.

Hashing an IPv4 address doesn't particularly protect privacy, given the limited address space.

In order to be gdpr compliant you need to be able to argue that you anonymized the data.

Hashing, possibly with a secret salt, is sufficient.