Ill. Admin. Code tit. 56 § 2720.35
Notwithstanding any provision to the contrary, a claimant's "last known address", as that term is used in the Act, is the last address provided to the Department by the claimant as provided in this Section. The address provided by a claimant on his or her most recent unemployment insurance claim application is the claimant's last known address until the claimant informs the Department of a change of address by telephone or in person at any local office, or through the internet at the Department's website, www.ides.illinois.gov. The telephone number for submitting a change of address can be found on the Department's website. Noting a change of address on an appeal or other correspondence is not sufficient to change one's "last known address".
Ill. Admin. Code tit. 56, § 2720.35