How Is Edge's IE Mode Set Up for a Legacy Intranet App?

What IE mode is, in one paragraph

IE mode is a compatibility feature of Microsoft Edge that renders specific, designated sites using Internet Explorer's engine inside an Edge tab. Microsoft's IE mode documentation describes it as a bridge for organizations with legacy sites that depend on old IE technologies — including ActiveX-hosted content — after Internet Explorer 11 itself was retired. Whether it can actually display a Silverlight app, and with what caveats, is its own question — covered in does Edge's IE mode run Silverlight. This page covers the other half: how IE mode gets turned on and pointed at an app in the first place.

The key idea: IE mode is opt-in, per site

Edge does not decide on its own to render a page in IE mode, and end users generally cannot force it permanently on their own. The design centers on a list of designated sites:

An administrator enables IE mode through policy. In managed environments, IE mode is switched on via enterprise policy settings, per Microsoft's configuration documentation.
A site list says which URLs get the legacy engine. Organizations maintain a list of the specific legacy URLs that should open in IE mode; everything else renders in modern Edge. This keeps the compatibility surface as small as possible.
Everything not on the list stays modern. That containment is the point — the legacy engine is exposed only where it's needed.

For a home user or an unmanaged PC, this means something important: if an old app only works in IE mode and no administrator has set anything up, there is no simple toggle that makes it a permanent fix. Edge offers limited user-level reload options, but they are designed as temporary conveniences, not a way to run a legacy app indefinitely.

What to ask your IT team

If you're the person who just needs the old app to open, the productive questions are:

Is IE mode enabled for our organization, and is this app's URL on the site list?
Does the app need anything beyond the legacy engine — a plugin runtime, an ActiveX control — installed on my machine as well?
Is there a planned end date for this arrangement, and what replaces the app afterward?

That last question matters because IE mode governs the browser engine, not the app's other dependencies. A Silverlight app also needs the Silverlight runtime on the machine, and Microsoft's lifecycle notice confirms Silverlight support ended in October 2021 — so even a working IE mode setup is running an unsupported runtime underneath, with the consequences discussed in the risks of keeping a Silverlight app deployed.

What to plan for if you're the admin

Treat an IE mode entry as a documented liability with an owner and an exit plan, not a permanent fixture:

Keep the site list tight. Every entry widens the legacy surface; add specific URLs, not broad wildcards, and prune entries when apps retire.
Track the unsupported components underneath. U.S. government guidance on patch management planning (NIST SP 800-40 Rev. 4) frames end-of-life software as risk to be inventoried, isolated, and scheduled for replacement rather than quietly tolerated — a good model for anything on an IE mode list.
Pair each entry with a migration line item. IE mode buys scheduling room for modernization; the durable fix for a Silverlight-era app is a rebuild on a supported stack, starting from what end of support actually means.

The bottom line

IE mode setup is an administrative act: policy plus a curated site list, controlled by the organization rather than the person at the keyboard. If a legacy intranet app matters to your work, the request goes to IT — and the follow-up question, always worth asking, is what the plan is for the day the compatibility bridge closes.