John Calvin Commentary 2 Peter 2:6

"and turning the cities of Sodom and Gomorrah into ashes condemned them with an overthrow, having made them an example unto those that should live ungodly;" — 2 Peter 2:6 (ASV)

The cities of Sodom. This was so memorable an example of Divine vengeance that when the Scripture speaks of the universal destruction of the ungodly, it commonly alludes to this as the type.

Therefore, Peter says that these cities were made an example. While this may indeed be truly said of others, Peter points out something unique here, because it was the chief and a vivid image; or rather, because the Lord designed that His wrath against the ungodly should be made known to all ages; just as, when He redeemed His people from Egypt, He set forth to us by that one favor the perpetual safety of His Church.

Jude has also expressed the same thing, calling it the punishment of eternal fire (Jude 1:7).