From Networkip, LLC v. FCC, 548 F.3d 116 (D.C. Cir. 2008):

Though agencies are entitled to deference, they may not retroactively change the rules at will. Indeed, that "[e]lementary considerations of fairness dictate that individuals should have an opportunity to know what the law is and to conform their conduct accordingly" has been well-established for "centuries." Landgraf v. USI Film Products, Inc., 511 U.S 244, 265, 114 S. Ct. 1483, 128 L. Ed. 2d 229 (1994). Anything less ought not to be dignified with the title of law. These "[t]raditional concepts of due process incorporated into administrative law preclude an agency from penalizing a private party for violating a rule without first providing adequate notice of the substance of the rule." Satellite Broad. Co. v. FCC, 262 U.S. App. D.C. 274, 824 F.2d 1, 3 (D.C. Cir. 1987).

[Footnote 5] The contrary notion of unknowable law is literally Orwellian. See, e.g., GEORGE ORWELL, ANIMAL FARM 102-03 (1946) (describing Squealer's ex post efforts to repaint the Seven Commandments to the pigs' whisky-bibbing benefit); see also Antonin Scalia, The Rule of Law as a Law of Rules, 56 U. CHI. L. REV. 1175, 1179 (1989) ("Rudimentary justice requires that those subject to the law must have the means of knowing what it prescribes. It is said that one of emperor Nero's nasty practices was to post his edicts high on the columns so that they would be harder to read and easier to transgress.").

At the same time, however, agencies are authorized to make policy choices through adjudication, and giving a decision retroactive effect is "not necessarily fatal to its validity." SEC v. Chenery Corp., 332 U.S. 194, 203, 67 S. Ct. 1575, 91 L. Ed. 1995 (1947). After all, "[e]very case of first impression has a retroactive effect, whether the new principle is announced by a court or by an administrative agency." Id. And, as is common with comprehensive regulatory schemes, often "every loss that retroactive application . . . would inflict on [one party] is matched by an equal and opposite loss that non-retroactivity would inflict on [another]." Qwest Servs. Corp. v. FCC, 379 U.S. App. D.C. 4, 509 F.3d 531, 540 (D.C. Cir. 2007). This case potentially stands at the pivot point between these competing principles.

There are "two conflicting modes of judicial review to agency interpretations," with "[o]ne longstanding line of [our] cases allow[ing] agencies to apply new interpretations of regulations retroactively," while another requires "revers[ ing] agency action where regulated parties do not have fair warning of the agency's interpretation of its regulations." Kieran Ringgenberg, Comment, United States v. Chrysler: The Conflict Between Fair Warning and Adjudicative Retroactivity in D.C. Circuit Administrative Law, 74 N.Y.U. L. REV. 914, 916 (1999). NET attacks with fair notice cases like United States v. Chrysler Corp., 332 U.S. App. D.C. 444, 158 F.3d 1350 (D.C. Cir. 1998); the FCC parries with retroactivity cases like Qwest.

When to apply which line of cases has not been resolved definitively by our precedents. We too leave for another day the question of how these two lines interplay, because under either one, NET loses.