Is there a way to categorise IP Addresses in the Web report?

You can create your own categories (if needed) under Web protection > Filtering options > Categories. Or you can use the existing ones

You can then re-categorise ip's, websites etc under Web protection > Filtering options > Websites.

You have to add each website although you can cut and paste from a list eg site A - E = category A eg really bad, then create another rule site F - J = category B = semi bad etc or simply re-categorise then using the existing UTM categories

Obviously a bit of a job to start with which will diminish with time.

Under reporting, the nearest you can get with web reporting is domains with categories. That will give you the domain visited with the category and amount of traffic.

If you want to go further, check out iView as the reporting is far superior on that.

The only time that you want to assign a URL to an allowed category is when you are certain that the URL is trustworthy -- reputable organization, devoid of active malware, with a purpose that is consistent with your organization's acceptable use policy.   Whitelisting large numbers of unknown websites defeats the point of having a web defense.   If you allow unknown sites at all, at least mark them as warned.

Louis is only half-correct about creating categories, and it points to one of the documentation problems.   UTM has two types of categories:   Websites are assigned to subcategories.   You can override the subcategory to which a website is assigned, but you cannot alter the subcategory list.   Users are allowed or blocked based on supercategories.  You can create or delete new supercategories.   A subcategory should only be assigned to one supercategory, but the UTM interface does not enforce this.   If you start rearranging the subcategory to supercategory mapping, you need to be careful to ensure that every category is assigned somewhere and no category is assigned twice, since either mistake could produce unexpected and undesired results.   The consequences of these mistakes are undocumented.

For your example, you cannot assign website 10.10.10.10 to a newly-created (sub)category called "Stuff_I_Approve".   You can assign it to the subcategory "Education" to cause it to be in an allowed supercategory, or assign it to "School Cheating" to cause it to be in a blocked supercategory.

The only time that you want to assign a URL to an allowed category is when you are certain that the URL is trustworthy -- reputable organization, devoid of active malware, with a purpose that is consistent with your organization's acceptable use policy.   Whitelisting large numbers of unknown websites defeats the point of having a web defense.   If you allow unknown sites at all, at least mark them as warned.

Louis is only half-correct about creating categories, and it points to one of the documentation problems.   UTM has two types of categories:   Websites are assigned to subcategories.   You can override the subcategory to which a website is assigned, but you cannot alter the subcategory list.   Users are allowed or blocked based on supercategories.  You can create or delete new supercategories.   A subcategory should only be assigned to one supercategory, but the UTM interface does not enforce this.   If you start rearranging the subcategory to supercategory mapping, you need to be careful to ensure that every category is assigned somewhere and no category is assigned twice, since either mistake could produce unexpected and undesired results.   The consequences of these mistakes are undocumented.

For your example, you cannot assign website 10.10.10.10 to a newly-created (sub)category called "Stuff_I_Approve".   You can assign it to the subcategory "Education" to cause it to be in an allowed supercategory, or assign it to "School Cheating" to cause it to be in a blocked supercategory.