Atleast per german court rulings, which at that time largely covered what is defined as PII under GDPR, say that the full IP is PII but the first three octets is not.
The last three octets absolutely could be in conjunction with some other pieces of data, though.
Of course, yes. Once you can identify a unique person it's personal data, even if you cut of octets. If you only log the IP for example for detecting duplicate accounts, it should be good enough.