CA Food & Agric Code Section 30951

It is unlawful for any person to own, harbor, or keep any dog over the age of four months, or to permit such a dog which is owned, harbored, or controlled by him to run at large, unless the dog has attached to its neck or leg a substantial collar on which one of the following is fastened:

(a)

A metallic tag which gives the name and post office address of the owner.

(b)

A metal license tag which is issued by the authority of a county, city and county, or any municipal corporation for the purpose of identifying the dog and designating the owner.